Bunuel
I need to know who’s to blame for all the mistakes I found.(A) I need to know who’s to blame for all the mistakes I found.
(B) I need to know whose to blame for all the mistakes I found.
(C) I need to know who to blame for all the mistakes I found.
(D) I need to know who is to blame for the mistakes I found.
(E) I need to know whose’s to blame for all the mistakes I found.
OFFICIAL EXPLANATION
Choice (A) is correct as written. Who’s is the contraction for who is and “who is to blame” makes sense here.
Choice (B) uses the possessive form, which is grammatically incorrect in this sentence.
Choice (C) changes the wording in an effort to fix the sentence but inadvertently creates a different error: who to blame is never correct. Blame requires an object and the proper form, then, would be whom.
Choice (D) simply gets rid of the contraction by spelling it out—not the right approach.
Choice (E) is madness, you must agree.
The answer is (A).