Question

Analyze the sentence to determine the main clause and the subordinate clause. Then choose the function of the subordinate clause.
"The man who wrote this new and popular book autographed copies of it at the bookstore."
Main clause:
Subordinate clause:
Subordinate clause functions as a(n):

Answer 1

The main clause here is The man autographed copies of it at the bookstore. This means that the subordinate clause is what is left - who wrote this new and popular book. 
The subordinate clause functions as an adjective, which means that it describes the subject the man. 

Answer 2

The subordinate clause of this sentence is: "who wrote this new and popular book."

This is an ADJECTIVAL subordinate clause.

Adjectival clauses modify nouns or pronouns of the main clause as an adjective would do it. These clauses are introduced by relative pronoun, adjectives or adverbs.

This clause is modifying the subject of the main clause -The man-, and is introduced by the relative pronoun "who."