Rewrite the sentence, instead of using ‘somebody’ or ‘they’, write a passive sentence:
Somebody is using the computer at the moment.
a. The computer is being used at the moment.
b. The computer is used at the moment.
c. The computer was used at the moment.
d. The computer has been used at the moment.

Hint: In the given question, the student has to find out which of the options indicates that the subject (computer) is the recipient of the action.

Complete answer:
Passive voice is the voice used to indicate that the grammatical subject of the verb is the recipient (not the source) of the action denoted by the verb.
Let us thus analyze the options given to us in this question -
Option (a.), 'The computer is being used at the moment', is correct as it indicates that the subject (computer) is the recipient of the action and uses the same tense as that in the active voice sentence.
Option (b.), ‘The computer is used at the moment', uses ‘past tense’ in the sentence. Therefore, option (b.) is incorrect as the tense used should be present continuous tense as tense is not changed when the sentence is changed from active to passive.
Option (c.), ‘The computer was used at the moment', refers to ‘past tense’. Therefore, option (c.) is incorrect as the tense used should be present continuous tense as tense is not changed when the sentence is changed from active to passive.
Option (d.), ‘The computer has been used at the moment', refers to ‘past perfect tense’. Therefore, option (d.) is incorrect as the tense used should be present continuous tense as tense is not changed when the sentence is changed from active to passive.

Note:
In this question, it will be helpful for the student to recall the concept of active voice. Active voice is the voice used to indicate that the grammatical subject of the verb is performing the action or causing the happening denoted by the verb.