Malcolm's primary role in Act 4 is to motivate Macduff to return to Scotland and seek vengeance on the tyrant who had Macduff's wife and children murdered. In a role reversal, Malcolm encourages the grieving Macduff to "convert his grief to anger" and see revenge on Macbeth. Macduff had come to Malcolm to encourage Malcolm to fight to regain the throne. Malcolm agrees, but when Ross brings the devastating news of the slaughter that took place at Fife, it is Malcolm who acts as the persuader, encouraging Macduff to return to Scotland to fight. The two agree to go back to Scotland and fight the villain "front to front."
Your options marked a, b, c are all viable in different degrees. It is true that Malcolm motivates Macduff in avenging the brutal massacre of his family, but the other options can not be altogether excluded. Orginally it is Macduff who migrates to England to convince Malcolm for an offensive against the tyrant. When Ross appears to break the news of the killings of Macduff's family, Malcolm ignites the fire of vengeance in Macduff. But, at the same time, Malcolm has to lead the offensive against Macbeth so that the prophecies made by the witches' apparitions come true. Malcolm does have a crucial role to play in getting the apparitions' equivocations translated to reality--the paradox of no man born of a woman ever killing Macbeth and the paradox of Macbeth's invincibility unless the Birnam wood comes to the Dunsinane hill. Again in the fifth and final act, the play moves fast to its catastrophe and resolution-- the defeat and death of the tyrant and the freedom of Scotland followed by the succession of the legitimate heir, Malcolm, to the throne. You can not decide upon Malcolm's primary role in act 4 without looking into what is going to happen in the final act.