Since the movie was released seventeen UFOs have been sighted in the

I try to explain with what I got. This is a case of Active voice vs passive. Here 'they' represents UFO's only but that clause should be in passive voice which is not present in choice B. Because UFOs have to be sighted by someone and it is the not the case that UFOs have sighted in the past ten years.

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Since the movie was released seventeen UFOs have been sighted in the state, which is more than had been sighted in the past ten years together.
(A) which is more than had been sighted (incorrect: tenses error)
Which is …………… in the past ten years: past ten years denote ended action and is denotes existence hence incorrect

(B) more than had been sighted (best option)
Had been denotes action before action or any action that was somewhere in the past

(C) more than they had sighted (incorrect: pronoun error and modifier error)
They doen’t have antecedent. More than they become modifiers and it is modifying state

(D) more than had reported sightings (incorrect: modifier error)
Without a relative pronoun, option D is a modifier and it modifies state.

(E) which is more than had reported sightings (incorrect: the same error as option A has)