Answer:
B. The phrase is a nonessential adjective clause.
Step-by-step explanation:
Nonessential adjective clauses are clauses that modify or describe nouns and whose information is not essential or necessary to the meaning of the sentence, and therefore, they are set off or framed by a punctuation, namely, commas, dashes or parenthesis (like in the sentence provided).
The other options are incorrect because the clause does contain a relative pronoun which it's "who," and it is not an essential adjective clause because it is not necessary to the meaning of the sentence.