Answer: B
Step-by-step explanation:
By using textual evidence, you can eliminate the other answers by filling in the blank.
Teaching is "the thing I want to do in the future".
Teaching is "what I want to do in the future" <--- Correct
Teaching is "which I want to do in the future"
Teaching is "whatsoever I want to do in the future"
B is correct because it is a clearer sentence.