Which of the following best characterizes a compound adjective?

A compound adjective is specifically characterized by its formation, which involves two or more words that are combined using a hyphen to create a single descriptive unit that modifies a noun. For instance, in terms like "well-known author" or "high-speed train," the hyphen indicates that the words work together to convey a specific meaning about the noun they are describing.

The other options do not accurately capture the essence of a compound adjective. For instance, while adjectives do describe nouns, the defining feature of a compound adjective is its hyphenated composition. Additionally, the notion of independent adjectives lacks specificity regarding how they function together, and an adjective modifying a verb is not relevant to the definition of compound adjectives, as adjectives primarily modify nouns. Thus, the defining characteristic of a compound adjective lies in its hyphenated form, making the correct choice clear.