Correct option is D. Santa Fe is one of the oldest cities in the United States, its adobe architecture, spectacular setting, and clear, radiant light have long made it a magnet for artists.
Here, the use of a comma is incorrect as both the parts of the sentence are independent clauses they should be connected by a semicolon. D rectifies this without creating any other error.
Sentences can be short or long, simple or complex. In order to have a correct, complete sentence, you really only need two things: a subject, and a verb that is conjugated to match the subject.
A subject and a correctly conjugated verb together, along with anything else that goes with them, form an independent clause. Don't worry, you don't need to know that term for the SAT! But it will be useful as we move forward for understanding sentences. An independent clause can stand on its own as a complete sentence because it makes sense as it is and expresses a complete thought.
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