The correct answer is C, indirect object. An indirect object is the part of the sentence that the action verb is being done to, or for. For example, in the sentence 'I threw the ball to my friend', 'friend' is the indirect object, because you're throwing the ball to your friend. The noun clause in this sentence is 'whoever calls on the phone', or 'whoever', and it is an indirect object because you're giving the directions to whoever calls on the phone.
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