Which of the following is an example of an adjective?

The choice of "Beautiful" as an example of an adjective is accurate because adjectives are words that describe or modify nouns, providing additional details about them. In this case, "beautiful" describes a quality of a noun, such as "a beautiful house" or "a beautiful sunset." It conveys information about what makes the noun appealing or pleasing to the senses.

To further illustrate, the other options include a word that functions as an adverb ("quickly"), which modifies verbs, adjectives, or other adverbs rather than nouns. The terms "run" and "thought" are both verbs and nouns, respectively, and do not serve the function of describing other words. Thus, "beautiful" stands out as the only option that fulfills the role of an adjective.