C works for me. The other 2 don't.
Student or Learner
Can one say
a. She was twice as smart as he thought of being.
b. She was twice as smart as he thought to be.
c. She was twice as smart as he thought he was.
I like (c). I have doubts about the other two.
In my example 'he' is someone else. The two 'he's refer to the same person.
You could change the sentence if you want, but you will change the meaning. I don't think your sentence would be wrong.