Descriptive words and phrases set off by commas, such as the italicized phrase in the following sentence, are
janet, a very good friend of mine, is coming to dinner on saturday.
a expletives c conjunctions
b appositives d injunctions

Your answer would be appositives (B).

Explanation:

Descriptive words and phrases set off by commas such as the italicized phrase in the following sentence, are called appositives. In the sentence above, the appositive phrase is the NP "a very good friend of mine" and it renames the noun "Janet".

An appositive is a noun or noun phrase that renames another noun right beside it. The appositive is nonessential as it can be removed from the sentence without altering the meaning of the sentence (1). That is why it is offset from the rest of the sentence with commas.

1) Janet is coming to dinner on Saturday

As you can see (1), the sentence has the same meaning without the appositive.

a

explanation: