Question

You had better ........... a doctor
(a) consult (b) to consult (c) consulted (d) consulting

Answer 1

The correct answer to the question is: You had better consulted a doctor.

This means that the speaker is telling the listener that he/ she should have consulted a doctor by then.

One of the other options given with the question are not matching.

So the correct answer as mentioned before is consulted.

Answer 2

Had better has similar meaning to should and ought.

You had better consult a doctor. (= You should consult a doctor.)

You had better get some rest. (= You should get some rest.)