Read the sentence. the musicians, who had been practicing for months, gave a stellar performance. which phrase is the adjective clause in the sentence?

Adjective clauses begin with words such as that, when, where, who, whom, whose, which, and why. I would say that it is the second phrase "who had been practicing for months".

it is the background information on the characters and setting explained at the beginning of the story. the exposition will often have information about events that happened before the story began. the exposition is often the very first part of the plot.