Read the sentence.

an expert in organ transplants, the chief surgeon of the hospital routinely spoke at conferences.
(an expert in organ transplants is underlined )
the underlined portion of the sentence is known as a(n)

a. appositive
b. relative clause
c. restrictive phrase
d. restrictive clause

A. Appositive    

Explanation:

An appositive phrase is a noun or a noun phrase that renames, identifies or gives further information of another noun that is right beside. This type of phrase can be either restrictive (when it's essential to the meaning of the sentence) or nonrestrictive (when it's not essential to the sentence, and therefore should be set off with commas, dashes or brackets when they are in the middle of the sentence).

In the sentence An expert in organ transplants acts as an appositive phrase because it provides further information of a noun that is right beside it: the chief surgeon of the hospital. Furthermore, it is also nonrestrictive because it is separated from the other noun by a comma, and it is not essential to understand the sentence.

c) problem and solution