Answer: have seen.
Explanation: a verb is a word or group of words that expresses an action performed by a subject in a sentence. A complete verb in a sentence, includes the main verb (the one that indicates the action) and all the helping verbs (the ones that extend the verb's meaning, or indicates an specific tense). In the given sentence the main verb is "seen" and the helping verb is "have" so the complete verb is "have seen."
