Which of the following is a complete sentence?

A. Since she was hungry, she had a snack.

B. Because I had not slept for two nights.

C. Although Henry was unusually tall.

D. Even when the temperatures improved.

The correct answer is A. Since she was hungry, she had a snack.

It is the only complete sentence because there is a main clause ("she had a snack"), and a subordinate (dependent) clause that hangs to it ("since she was hungry"). All the other examples are subordinate clauses, and they can never function on their own, as they don't provide the complete piece of information.