Which of the following sentences is punctuated correctly? let's go to the beach since you love to swim let's go to the beach, since you love to swim let's go to the beach since you love to swim.

C

Explanation:

The sentence which is correctly punctuated is the last one. First of all it's not necessary to have a comma before the "since", so the second  one is discarded. Secondly as the first sentence doesnt have the final stop, it's also discarded because without it, it cannot be considered a complete sentence. Finally, the third sentence is the only one which has the full stop at the final of it and it also doesn't have a comma in the middle of it, so it's the correct answer.