Answer:
C) Alex's computer (which he had owned for three years) suddenly went
missing after school.
Explanation:
Parentheses are the punctuation marks that are used in the sentences to add additional information. The information is related to the meaning of the sentence. The information enclosed in the parentheses can be in single word or in phrases or in long sentences.
From all the given options, the sentence in option C has used the parentheses correctly. The phrase 'which he had owned for three years' is the additional information about the computer the Alex had.