B). I started to do voluntary work three years ago.
The second option correctly transforms the given sentence into the past using past indefinite tense. Past indefinite tense is employed to denote the actions that occurred or happened at a particular point of time in the past('started to do voluntary work three years ago'). It is framed using 'subject + V2 + object.' While present perfect tense is employed to denote an action that began in past but still progressed till now. Such actions carry their effects in the present which cannot be possible in the given context as the action took place entirely in the past. Thus, <u>option B</u> is the correct answer.
Interrogative pronouns are what, who/whom, which, and whose.
An interrogative pronoun tells us which piece of information is missing. In a sentence - <em>Who is your best friend?</em>, interrogative pronoun<em> who </em>indicates that the piece of information we are looking for is the best friend's name.