Answer: A) to enrich.
Explanation: an infinitive is the basic form of a verb, without an inflection binding it to a particular subject or tense. It is preceded by the particle “to,” and can serve as an adjective, an adverb, or a noun. In the given sentence, the part that contains an infinitive is "to enrich" because it has the particle "to" followed by a verb. In phrases like "to the delight," "to Tuesday mornings," and "to Thursday afternoons" the particle "to" is acting as a preposition, therefore they aren't infinitives.