You ___ better be careful not to miss the train! A.

You ___ better be careful not to miss the train! A. Would B. Should C. Had D. Did

You had better be careful not to miss the train! 

The correct answer is C. Had

Had better is always followed by a verb in the infinitive without to:

You must or should be on time. 

Had better is ALWAYS formed from the auxiliary verb have in the past simple (has better or will have better do not exist!).