It's education that makes us who/what/where we are.

[Andy Lin]

Source from Lai's ENGLISH GRAMMAR authored by Shixiong Lai.

It is education that makes us ____ we are.
(A) who (B) whom (C) what (D) where

Author said C is correct answer because it's a Compound Relative Pronoun.

I wonder why A and D are not correct answer?

 

[jutfrank]

(A) and (C) are both grammatical, so the author's reasoning isn't right.

The best answer is (C) because of meaning, not grammar.