• An adverb clause modifies a verb, another adverb, or an adjective.
• In the sentence, we need to modify the verb "comes."
• "Whenever" indicates time and expresses repeated occurrence.
• "Wherever" indicates place, "as" is a conjunction to introduce a reason, and "when" is a general time adverb but doesn't necessarily imply repetition.

Therefore, "The doctor always comes whenever he is sent for" is the most natural and grammatically correct way to complete the sentence using an adverb clause of time.