There was a question on the test that I was not sure which option was correct. The question is
Fill in the blank choosing the most appropriate word.
Duke is not ( ) we think him to be.
who / how / where / what
It seems to me that "how", "where", "what" could go into the blank, as I thought we could say;
- We think him to be nice. --> an adjective "nice" can be replaced with "how."
- We think him to be a doctor. --> a noun "a doctor" can be replaced with "what."
- We think him to be at home. --> an adverbial phrase "at home" can be replaced with "where."
And "who" sounds funny in an ordinary situation because we already know it is Duke. I chose "what" in the end.
Will someone please explain to me which one is correct, together with some explanation?