The correct option is A was also seen
The first sentence is in past perfect continuous tense.So the construction of the given sentence which is related to the first sentence should be in past tense, and 'a pair of sunglasses' is a collective noun, which denotes a singular subject. So, option D is correct. Option A,B,C are in simple present tense , so they are inappropriate for the sentence. the completed sentence:a pair of sunglasses was also seen in it.