Which word is usually used as an adjective?

The word "tall" is used as an adjective because it describes a characteristic or quality of a noun, specifically referring to height. Adjectives are words that modify nouns and provide more information about them, such as their size, color, shape, or other attributes. In this case, "tall" describes the physical height of a person, object, or other entities, making it a proper adjective.

In contrast, the other words in the choices serve different functions. "Run" is primarily a verb, denoting an action. "Quickly" is an adverb, which modifies verbs, adjectives, or other adverbs rather than nouns. "Under," while it can be used as a preposition or an adverb, does not function as an adjective unless it is part of a larger descriptive phrase. This differentiation highlights the role of adjectives as descriptive modifiers in language.