The correct option is C will have acted
The sentence indicates that an action will have been completed (or perfected) in the future. This is indicated by the phrase "By the time he meets" which shows the completion of an action at one point of time 'by the time'. Hence this sentence is of the future perfect tense and the action should be of the form will+have+past participle, where the past participle is a form of a verb that ends in -ed and is used in perfect tenses. Option D is of the future perfect tense (will+have+acted) which is placed in the sentence, hence option D is correct. 'Had acted' is in the past perfect tense, it indicates an action completed in the past. 'Has acted' is in the present perfect tense and indicates a link between the past and the present, the action began in the past and has been completed in the present. 'Will be acting' is in the simple future tense, it indicates an action that will take place in the future, but does not mention its completion. Since all these are not in the future perfect tense, options A, B and C are incorrect.