In the sentence "It took less time to drive from one city to the other than they had anticipated", the answer that correctly describes the bolded word is D:
The word less is an adverb describing to what extent.
In this sentence, the word less functions as an adverb; this means that it modifies or qualifies an adjective, a verb (in this case), or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc.