The correct answer is Option D.
WOULDN'T is the negation of WOULD.
WOULD expresses a desire and wish.
In the given sentence, the speaker is expresses his/her desire to not become a teacher.
Thus WOULDN'T is the correct answer.
The remaining options are incorrect because:
CAN'T expresses inability.
WON'T is the contraction of will not.
COULDN'T is the past tense of CAN'T.