Answer: The correct description for the words in bold is C. clause.
Explanation: The words in bold ("as more young people live independently before marriage") represent a clause. More specifically, they make up a dependent clause. Although a dependent clause contains a subject and a verb, it cannot stand alone as a sentence since it does not present a complete idea. Moreover, this type of clause is always introduced by a subordinating conjunction. For example, "As more young people live independently before marriage" does not express a full idea. Furthermore, this clause has been introduced by the subordinating conjunction "as", which indicates a cause.