Question

Which sentence contains an adverb clause? A. California has three of the top ten most populated cities in the United States. B. When you return from California, take a taxicab from the airport. C. The state of California is also called the Golden State. D. Yosemite National Park, in central California, covers nearly 1,200 miles of mountainous terrain.

Answer 1

The answer to your question would be that the sentence that contains an adverb clause is the following one: When you return from California, take a taxicab from the airport. That is, the correct option would be B.

An adverb clause is a dependent clause that functions as an adverb in a sentence. As they are dependent clauses, they must have a subordinating conjunction connecting them to the rest of the sentence.  In this case, the subordinating conjunction "when" is the one that introduces the adverb clause. It gives information about the time in which you should take the taxicab from the airport.  

Answer 2

 A. California has three of the top ten most populated cities in the United States 
this contains an adverb clause
so correct option is A 
hope it helps