In The Crucible, Danforth says that if the accused are innocent they have nothing to fear because:
a he wants the people to believe the court is infallible
b he wants to quell dissensions and promote compliance
c the court will help the accused to prove their innocence
d he wants to divert attention from the need to sacrifice innocent people for the greater good in the war against evil
Actually, if we look very carefully at Act III of this play, we can see that all of these answers are correct. This of course can be shown when Francis presents Danforth with the deposition of ninety-one friends and neighbours who have signed it to say that Rebecca, Martha Corey and Goody Proctor are all good, Christian women. When Danforth agrees that each one of those ninety-one people must be brought into court for questioning, he responds to Francis Nurse's concerns that he has brought evil upon these people with the following words:
No, old man, you have not hurt these people if they are of good conscience. But you must understand, sir, that a person is either with this court or he must be counted against it, there be no road between. This is a sharp time, now, a precise time--we live no longer in the dusky afternoon when evil mixed itself with good and befuddled the world. Now, by God's grace, the shining sun is up, and them that fear not light will surely praise it.
You can see how this quote can be used to indicate that each answer is correct. Perhaps the most relevant if you have to pick one would be the last option, as this quote clearly identifies that Danforth beleives himself to be engaged in a spiritual war between good and evil.
To answer the question accurately, one needs to assess what the judge says, his actions in this regard and the events which unfold as a result of these. As it is, the judge's words have an ironically hollow ring to them, since it is quite clear that the court's approach, and especially his, is that one is guilty until proven innocent. The onus is solely on the accused to prove themselves innocent of the charges against them.
(a) It is clear that the judge wants to convince others that the court cannot make any mistakes for, as he continuously stresses, it essentially has a divine purpose and is, in effect, acting as the voice of God in its desire to rid Salem of the forces of evil which, he believes, are waging a war against whatever is good and holy in the village. One cannot, therefore, question the authority of the court or its integrity for, in doing so, one is also questioning the divine.
(b) When Giles Corey wants to address the court to prove his wife's innocence and introduce testimony about Mr Putnam's greed for property by using his daughter to accuse others, he is admonished and later incarcerated by the judge for causing dissension. He also later warns John Proctor, on reverend Parris' insistence, to not attempt usurping the court's authority and try to overthrow proceedings. In guaranteeing that the innocent have nothing to fear, he is attempting to encourage them to consent to his power and allow him to make decisions no matter how arbitrary they may seem. In other words, they should completely agree with him and not dispute his decisions.
(c) Clearly, judge Danforth wants to provide this assurance, but his actions display completely the opposite. Whenever the accused wish to present evidence, their motives are questioned. When they want to present testimony and provide names, the court requests the arrest of witnesses so named. Judge Danforth also repeatedly states that he has no reason to doubt the girls' testimony since he sees them as acting on God's instruction. They are blameless. This indicates an obvious bias in favor of the girls.
(d) Throughout the trials, it is the judge's contention that they are fighting a desperate war against the forces of evil. It should, therefore, be obvious that innocents will die in the process. The judge, however, does not, at any point, accede that the innocent would be any of the accused. It is only the victims of those accused which he deems innocent and their innocence is the sacrifice. The judge delivers harsh judgement but he emphatically believes that everyone who has been accused must, as a rule, also be guilty.
Of the four reasons, this is probably the only one which is not valid as a basis for the judge extending the assurance that the innocent have nothing to fear.
On the whole, the citizens of Salem are in terrible fear and feel threatened by judge Danforth's presence, as reverend Hale states in Act 3:
We cannot blink it more. There is a prodigious fear of this court in the country -
Judge Danforth, however, sees the remark as an attack on his integrity and his ability to judge and later uses a clever argument to prove the point that he is correct in his assessment of the accused.
...witchcraft is ipso facto, on its face and by its nature, an invisible crime, is it not? Therefore, who may possibly be witness to it? The witch and the victim. None other. Now we cannot hope the witch will accuse herself; granted? Therefore, we must rely upon her victims - and they do testify, the children certainly do testify...
It is ironic, though, that he does not use the same rationale when referring to the accusers. They are, by virtue of their accusations and testimony, innocent whilst he has determined that the accused are definitely witches only because they have been accused of being so. In the end, it is more his arrogance and self righteousness that causes the deaths of so many innocents.