The fire alarm sounded, and two engines raced to the scene. This is the compound sentence because if you split the sentence into two at the "and", two complete sentences are formed. That is also why a comma is after this "and" because it connects two complete sentences.
1 is wrong, 4 is wrong. The answer could be either 2 or 3, but it is most likely 2, as the call to action generally takes place at the very end of an essay if it's written correctly.
