Hi Nicole,
It's good that you know better than to assume causation. Certainly if you are provided information about a correlation or given a correlation coefficient you cannot make any assumptions regarding causation. Not because there cannot be a causal effect, but simply because we don't have enough information to make that call.

In this case we are just given the description of a study, and asked to draw a conclusion. The correct answer is not A, because it is very assertive in saying the effects seen in the study are a direct cause and effect, which is more that we are able to claim with confidence with the information given. However, the correct answer, option C, claims there is evidence that inadequate sleep can cause hunger. Which is exactly what the study showed. Participants caused to have inadequate sleep showed an increased hunger response. While this may not be enough information to assert a definitive causal effect, it is evidence for that claim.

It's also worth noting that you are always looking for the best answer, and using a process of elimination can help you find a correct answer. Options B and D refer to correlation. But if you think about those options and the way they are phrased, they certainly aren't better answers. To be strongly correlated both need to trend in a certain direction if either variable changes. But does it make sense to have a correlation between hunger and inadequate sleep? The study shows that hunger increases as inadequate sleep increases, but we can't say that as hunger increases so does inadequate sleep. We can use common sense to know that how hungry you get throughout a day doesn't impact how much sleep you got the previous night. So both of these answers drawing a correlation between the two variables can be eliminated.

I hope this helps!
Nicole