Answer: D) Complete sentence.
Explanation: The given sentence ("it was a dark and stormy night") is an example of a complete sentence, because it has a subject ("It"), a verb ("was"), and complements ("a dark and stormy night"). Also, it isn't a subordinate clause, because it has complete meaning by itself (it doesn't depend on another clause). So the correct answer to the question is the corresponding to option D: complete sentence.