An action in a sentence can be represented in two ways, namely active and passive voice.

The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in the active voice.

The correct answer is B) He was seen picking up a gun by someone.

Option B is correct as the subject of the given sentence (He) comes in the place of the object (someone) and vice versa, the same tense is used (simple past tense), appropriate helping verb (was) is used, and the past participle of the main verb ( see) is used.