Fill in the blank with the most appropriate option:
A verb is used to express an occurrence, action, or state. It is the grammatical center of the predicate in a sentence.
An auxiliary verb is a verb which is used in the formation of tenses, moods, tones, etc.
Option A is correct as planned indicates the use of had to for expressing past necessity. Also, the subject is singular.
The other options are wrong as no other tense can be used here.
The correct answer is A) had to