Concept:The present perfect tense is used with "since" to show an action or state that started in the past and continues to the present.Explanation:The sentence is: "I ____ him since my childhood."Here, "since my childhood" indicates a time from the past continuing until now.With "since", we use the present perfect tense: "have/has + past participle".The verb "know" is a stative verb, so it is not used in continuous form like "knowing".The subject is "I", so we use "have known".Answer:C. have known