Read the sentence: an army of ants quickly across the sand. which verb correctly completes the sentence? a) has marching b) march c) marches d) marching

The verb which correctly completes the sentence is the following one:

c) marches.

An army of ants marches quickly across the sand.

In the sentence, the subject is "an army of ants", that is, a single army, therefore singular. Since the sentence is in simple present tense and is affirmative, as well as the subject is in third person singular (an army of ants marches= it marches), the rule of adding "s", "ies", or "es" to the verb applies. Since the main verb ends in "ch", "es" must be added to it.