An adjective phrase can be a single adjective or a group of words built around____

The correct answer is: Adjective phrase

An adjective phrase can be a single adjective or a group of words built around a single adjective.
Or, in other words, An adjective phrase always has an adjective acting as the head. The adjective phrase may also contain words or phrases before or after the head (modifiers and complements):

for example: The students were really bored with the film.

Source: Cambridge Dictionary Grammar reference.