Topic: Interchange of Parts of Speech,

Best Answer Explained in Below-:

The question asks for the adverb form of the noun "heart." In English, adverbs are words that modify verbs, adjectives, or other adverbs, often indicating manner, degree, or frequency. In this case, the adverb form related to "heart" is "heartily" (Option D).

"Heartily" derives from the adjective "hearty," which expresses warmth, enthusiasm, or sincerity. The suffix "-ly" is commonly added to adjectives to form their corresponding adverbs, which is why "hearty" becomes "heartily." For example, one might say, "She greeted him heartily," meaning her greeting was warm and enthusiastic.

Let's evaluate the other options:

- Option A, "heart," is the base noun and does not function as an adverb.
- Option B, "hearten," is a verb that means to make someone feel more cheerful or confident; it is not an adverb.
- Option C, "hearty," is an adjective describing something full of warmth or zest, but again, it does not serve as an adverb.

Thus, "heartily" is the only correct answer, as it effectively modifies verbs, allowing us to express actions done in a heartfelt and spirited manner.