The correct option is
B was/was raining
Option B is the correct answer because both "was" and "was raining" are in the simple past tense, as "was" is the past tense of "is", hence it is correct.
Option A is incorrect because "is" dark is correct, but then, the following statement should be "it is raining" , then the statement would have been correct as the entire statement should be in the same tense, past or present.
Option C is incorrect because "are" is used as a plural form, while this statement should be made in the singular form, as they are talking about a place, hence "it is dark" is correct.
Option D is incorrect because then the statement would remain incomplete.
Option E is incorrect for the same reason as D.