Surely the lost child must have been found by now
Answer: B
The sentence is of present perfect tense. It is also in passive voice. We can say this because the sentence has ‘have been’ in it. So its transformation into active voice would be ‘has/have + past participle form of the verb’. Also, the vague pronoun (someone) is missing in the passive tone, we’ll add it back in active. Therefore, option (b) would be the correct choice