Which of the following is an example of an adverb?

The selection of "kindly" as the correct answer is appropriate because it functions as an adverb, which is a word that modifies verbs, adjectives, or other adverbs. In this case, "kindly" can describe how an action is performed—indicating that something is done in a kind manner. This highlights its role in providing additional context to the action.

Other choices do not serve as adverbs. "Friend" refers to a person and functions as a noun. "Tree" also serves as a noun, representing a type of plant. "She" is a pronoun, used to refer to a specific female individual, and does not provide the nuance of action or manner that an adverb would communicate. Thus, "kindly" stands out as the only adverb among the given options.