The correct answer is B. He was seen picking up a gun by someone.
A. He was seen pick up a gun by someone is incorrect because the verb “pick up” is a phrasal verb, which means that it is made up of two words: the verb “pick” and the preposition “up.” Phrasal verbs are always treated as a single unit, so the past participle of “pick up” is “picked up,” not “pick.”
C. He was seen when he was picking up a gun is incorrect because it does not specify who saw him picking up the gun. The sentence could be interpreted to mean that he was seen by anyone, or that he was seen by a specific person.
D. He was seen by someone pick a gun is incorrect because the verb “pick” is a transitive verb, which means that it takes a direct object. The direct object of the verb “pick” is the gun, so the sentence should read “He was seen picking up a gun.”
In conclusion, the correct answer is B. He was seen picking up a gun by someone.