Which of these words is an example of an adjective?

The word "blue" is an example of an adjective because it describes a noun by providing information about its color. Adjectives are words that modify nouns or pronouns, giving more detail, such as size, shape, color, or quality. In this case, "blue" tells you what kind of color something is, making it a descriptive word that enhances our understanding of the noun it modifies.

In contrast, the other options provided are examples of adverbs or verbs. "Quickly" and "happily" describe how something is done and modify verbs rather than nouns, while "run" is a verb that indicates an action. This distinction highlights the unique role of adjectives in language, which is primarily to add descriptive qualities to other words.