The infinitive is the most versatile verb form because it can be used asthe infinitive is the most versatile verb form because it can be used as
a. a verb, a noun, an adjective, or an adverb.
b. a verb, a noun, an adverb, or an interjection.
c. a verb, an adverb, or a conjunction.
d. a verb, an adjective, or a preposition.

I think the correct answer would be A. The infinitive is the most versatile verb form because it can be used as a verb, noun, adjective and even an adverb. An infinitive verb is classified as a non-finite verb wherein it could be used as the principal verb in a sentence. The infinitive verb form of a verb is commonly preceded by the word "to".  For instance: to run, to dance, to swim, to sleep etc. An example of an infinitive verb as a noun is: To swim a mile is her greatest dream. The infinitive form of the verb is "to swim"