Question

Which is a complete sentence?

A.
A black cat sat on the windowsill.

B.
During the drive to the waterfall.

C.
Just as the doorbell rang for the third time.

D.
To make a long story short.

Answer 1

A complete sentence needs a main clause with a subject and verb, forming a complete thought.

Just looking at these you can see that only A has a complete thought, with both a subject AND a verb. So, your answer is A.

Answer 2

A im sure

Answer 3

It is definitely A.

Answer 4

A.

Answer 5

B. is a sentence fragment

Answer 6

A