The correct option is C Subject + will have been + present participle
The tense of a sentence denotes a time, whether it is the time now (present), time then (past) or the time to come (future). The verb of a sentence has most to do with the tense. The Future Perfect Continuous Tense describes an action that is still continuing even after a certain point of time in the future. It's of the form will have been + present participle (verb + ing, as the action is still continuing) as seen in option D, hence it's correct.. In option A, the error is in 'being', which itself is a present participle, the correct form is 'been', hence option A is incorrect. In option B, the error is in 'past participle', because it shows that an action has completed, whereas the action is in progress, hence option B is incorrect. A participle is a verb form, with an -ed or -ing added to the verb (completed, completing) to form a tense. In option C, it isn't specified which tense form of the participle is used, and it isn't optional, only the present participle is added in a future perfect continuous tense, hence option C is incorrect.