The correct answer is: B. He was seen picking up a gun by someone.
The other options are incorrect because they do not correctly convey the meaning of the sentence.
Option A is incorrect because it uses the past participle “picked” instead of the present participle “picking.” The past participle is used to form the passive voice, which is not appropriate in this sentence.
Option C is incorrect because it uses the present tense “is seen” instead of the past tense “was seen.” The present tense is not appropriate in this sentence because it implies that the action of seeing is ongoing.
Option D is incorrect because it uses the preposition “by” instead of the preposition “picking.” The preposition “by” is used to indicate the agent of an action, while the preposition “picking” is used to indicate the action itself.
In conclusion, the correct answer is: B. He was seen picking up a gun by someone.