The correct answer is: Physics was my best subject at school but when I started college, my grades in physics came down.
The verb “was” is used to describe a past state or condition. In this case, the speaker is describing their past state of being, which was that physics was their best subject. The verb “started” is used to describe the beginning of an action. In this case, the speaker is describing the beginning of their college career. The verb “came” is used to describe a change in state or condition. In this case, the speaker is describing the change in their grades in physics, which went down after they started college.
The other options are incorrect because they do not use the correct tense of the verb. Option A uses the present tense, which is incorrect because the speaker is describing a past event. Option B uses the past perfect tense, which is incorrect because the speaker is not describing a completed action. Option C uses the present progressive tense, which is incorrect because the speaker is not describing an ongoing action. Option D uses the past progressive tense, which is incorrect because the speaker is not describing an action that was happening at the same time as another action.