(D) represents a key piece of information because if sea turtles are outside the area where the smell is perceptible, then the hypothesis that the smell stimulates the turtles no longer makes sense since they cannot detect it, whereas (C) only tells us whether many beaches have been destroyed, but it is impossible to make a proper inference regarding the impact of many beaches being destroyed without further information. Even if (C) is true, does it mean the smell is no longer detectable? Does it mean that smell is still detectable but the turtles that were born on these beaches cannot return there for other reasons but the hypothesis is still true for the turtles that were born on undisturbed beaches? Since (D) alone is insufficient to prove or disprove the hypothesis, we can rule it out as the correct answer.
