Which is the subject of this sentence? Along the way, stop and take in the breathtaking views. A. stop B. views C. an understood you D. way

The correct answer here is: you. Specifically, it's "an understood you" - answer C. It is called like this, because the word "you" does not actually appear in the sentence, but it's there nonetheless, it's "understood" that it's there. In this sentence, the subject is "you" and the verb is "stop and take".