The given sentence is the case of vague pronoun.
The subject of the Passive Voice is changed into the object of the Active Voice. Also, the object of the Passive Voice is changed into the subject of the Active Voice.
The removal of the "to be" verb ('been' in the given sentence) and the preposition "by" is necessary while changing the voice of the sentence from Passive to Active.
In Active voice, the introduction of "Somebody" is necessary for these types of sentences.
For example:
I have been robbed. (Passive)
Somebody has robbed me. (Active)
The sentence in the Passive voice is in Present Perfect tense (has + been + V3). So, the sentence in the Active voice should be in Present Perfect tense (has + V3).
The options except (a) don't follow the correct grammatical format of the Active Voice.
Therefore, the correct answer is option (a): Someone has completed the task.
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