Answer:
D. If I want to be a doctor, I have to do well in science class.
Explanation:
In the sentence given in the question, the main purpose or point of the sentence is -
in order to become a doctor, one needs to do well in the science class.
Now, according to the cause and effect relationship of the sentence, the cause is the "doing well in science class" part while the effect will be "becoming a doctor". For the latter to happen, the former has to be done first.
Thus, to rewrite the whole sentence in a correct way, it has to be written as
"If I want to be a doctor, I have to do well in science class", which correctly implies the cause and effect sequence of the proposed purpose of the sentence.
