Topic: Interchange of Parts of Speech,

Best Answer Explained in Below-:

The correct answer to the question "The adjective of the word 'authority' is" is A) Authoritative. To understand why this is the case, we need to clarify the function and meaning of each option provided.

1. **Authoritative (A)**: This is indeed the adjective form of "authority." It describes something that is regarded as definitive and reliable, often used to denote power or expertise in a certain area. For example, an "authoritative source" conveys trust and credibility.

2. **Author (B)**: This term is a noun that refers to a person who writes. While an author may have authority in their field, this word does not function as an adjective.

3. **Authorise (C)**: This is a verb meaning to give permission or approval, and it does not describe a characteristic or quality, which is what adjectives do.

4. **Authoritatively (D)**: This is an adverb, which modifies verbs, adjectives, or other adverbs and describes the manner in which an action is performed, but it does not serve as an adjective.

In summary, since "authoritative" describes the nature of someone or something having authority, it is the correct answer as the adjective form of "authority."