His answer to all my questions was

consistently "eight more hours," no

matter how late in the day it became.

A. was consistently

B. were consistently

C. were consistent

D. was consistent

A

Explanation:

The subject of the sentence is "answer," a singular noun. (No, "questions" is not the subject because it is contained within the prepositional phrase, and subjects are never located there - - - even though you might be fooled into thinking that since that noun just happens to be located next to the verb). Therefore, the verb must also be singular for subject-verb agreement. "Was" is a singular verb.

"Consistently" (an adverb) is appropriate, not "consistent" (an adjective).