Test 7 Section 4 - #19 (page 776)

This is a Verb Agreement question.  The verb here is "is." What is the subject?

The subject is not "mountain range."  That's where the sources of the river are. One of the SAT's favorite tricks is to put the subject after the verb in hopes of tricking you.

The subject, here, is sources, which means our verb should be are, not is.  You could rewrite this sentence to say (prepositional phrase crossed out to make the subject/verb relationship more clear):

The sources of a river whose course was not fully mapped until this century are in the foothills in that large mountain range.

The answer is (A).