What kind of phrase or clause is in parenthesis in the following sentence?

news corp has been ramping up its sports digital portfolio (which is a way to broaden its audience and entice readership.)

adverb clause
none of these
prepositional phrase
adjective clause

Adjective clause

Explanation:

Adjective clauses are dependent clauses (group of words having a subject and a verb and that do not have a full meaning on its own) that modify, describe or adds further detail to nouns and that often start with a relative pronoun (which, who, whom, whose or that) or a relative adverb (when, where, or why).

In the sentence, "which is a way to broaden its audience and entice readership" is a clause because it has a subject: "which" (reference to the subject "portfolio") and a verb: "is", and it is also an adjective clause because it adds further detail or information to the noun "sports digital portfolio."  

oka

i think i seen this before

what does the teacher mean