Question

Peter didn't know ___ was his.
whichever
which
whomever
who

Answer 1

which... it is the only one that makes sense in the sentence.

Answer 2

The last two answers are wrong automatically because they make no sense what so ever. The first answer is wrong because putting it in that sentence doesn't sound correct. So the answer would be B. Peter didn't know which was his.

Hope this helps! :)