The correct answer is "the boy in the red shirt"
Explanation:
The purpose of an appositive phrase is to describe or add information about a noun, for example, a person, place, object or animal. Due to this, appositives can include adjectives or other nouns that provide further details. Moreover, appositive phrases are considered as non-essential and are often enclosed by commas. In the case of "Alex, the boy in the red shirt, is the captain of the football team" the appositive phrase is "the boy in the red shirt" because this describes the noun "Alex", also, this section is enclosed by commas which shows the phrase is non-essential.
