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 D)Someone has picked his pocket.

Option D is correct as the given sentence is in the passive voice and to transform it into the active voice, the subject of the given sentence (His pocket) comes in the place of the object(who has picked his pocket- 'someone') and vice versa. Also, as 'has been' is used in the sentence and 'has' shows that the transformed sentence must be in the present perfect tense.

Options A, B, and C are wrong as 'they' cannot be used in the place of the subject in the transformed sentence, the sentence structure is wrong(also it's tense), and the subject is not mentioned respectively.