Select the sentence in which 'legal' is used as an adjective.

The sentence where "legal" is used as an adjective is the one stating that he is a legal resident of the United States. In this context, "legal" describes the noun "resident," indicating that his status as a resident conforms to the law. Adjectives serve to provide more information about nouns, and here "legal" specifies the type of resident, implying that he has lawful status.

In the other sentences, while "legal" appears, it's often serving different grammatical roles. For instance, "legal" in the phrase “it is legal to wear green” functions in a way that expresses the permissibility of an action, but does not directly modify a noun in the same manner as "legal resident." Understanding the different uses of "legal" helps clarify how adjectives function in a sentence.