Pay close attention to the structure of the sentence. The meaning of the sentence is important here as well. Structurally, it's possible that "adapting heart muscles" should be parallel to "relatively slow, easy running," but that doesn't make logical sense. Instead, changes to the heart muscle is being likened to adjustments in and around muscle cells.
Thus, "adapting heart muscles" should be similar in structure to "adjustments in and around the muscle cells." The preferable construction, then, is "adaptations to the heart muscle" in (C) or (D). "Adaptations" is structurally and logically similar to "adjustments."
(D) is incorrect as it moves "relatively" to a different position. In the original sentence, "relative" modifies the slowness of the running; in (D), it modifies "excellent," which is not only logically incorrect, but inelegant as well. Choice (C) is correct.