Either a subordinating conjunction or a relative pronoun may introduce a(n)
independent clause
verbal phrase
prepositional phrase
subordinate clause

Either subordinating conjunction or a relative pronoun may introduce a subordinate clause.

Explanation:

A clause which provides additional information in a sentence is said to be a subordinating clause or dependent clause. A subordinating clause does not have an independent stand in the sentence. It either modifies or provides a component in the sentence.

c) in the address bar

explanation:

this is the only place where url's may be found or entered in to successfully get to a web page.