What reasons does Antonio give Bassanio for not being afraid of the contract?
I assume that you are asking about what happens in Act I, Scene 3. In that scene, Antonio makes the deal with Shylock where Antonio will borrow money to help Bassanio. If he can't pay it back, Shylock can have a pound of his flesh. Bassanio worries about this, but Antonio basically just laughs at his fears.
Antonio gives one major reason for dismissing Bassanio's fears. He says that there is plenty of time left before the money has to be paid back and that his ships will be back a month before the loan is due. He is saying that, when his ships come back, he will have plenty of money and there will be no danger of having to give up the pound of flesh. There are two quotes that show this:
Why, fear not, man, I will not forfeit it;
Within these two months,—that's a month before
This bond expires,—I do expect return
Of thrice three times the value of this bond.
Come on; in this there can be no dismay,
My ships come home a month before the day.