The Present Perfect tense is formed with the auxiliary verbs <em>has </em>or <em>have</em>, followed by the past participle. It is usually used to talk about experiences at an indefinite time in the past.
The rest of the options are incorrect because the verb <em>volunteer </em>in the past form is <em>volunteered</em>; in the past perfect it is <em>had volunteered</em>, and in the present simple tense it would be <em>(Colin) volunteers.</em>
