Hi Renee,
I like to think of these types of questions as asking about the "3 C's"--causation, correlation, and coincidence.
Causation means that one thing actually caused the other. For example, studying hard for an exam might cause better grades.
Coincidence is when things just happen to look related when they're really not. For example, it might be a coincidence that you spun around three time and said, "Abracadabra" and you got a good grade on your test. Those things aren't really related at all.
Correlation is the thing in between that is a little trickier to understand. When two things are correlated, it means that there is a relationship of some sort. When one goes up, the other does, too. Or when one goes up, the other goes down. But the tricky part is that people often assume there's a causal relationship when in actuality there is a third thing that is causing BOTH of the other two to happen. Here's an example: Number of ice cream cones purchased and shark attacks are correlated... when one goes up, so does the other. Does that mean buying ice cream causes sharks to attack? Of course not. But it's not just a coincidence, either. There is a third factor that predicts both: summertime. In the summer, people buy more ice cream cones, and they also swim in the ocean more, which can lead to more shark attacks.
In your question, choice D isn't talking about the relation between two things at all. Choice A is also only talking about one thing--reading comprehension--even if they are checking that one thing in two different groups. Both choices B and C are indeed comparing two things. However, choice B is more likely to actually be a causation situation--frequent religious service attendance IS probably caused by religious commitment. Choice C is the one that is simply a correlation. In other words, being rich doesn't CAUSE you to have more medical issues. Rather, if you are rich, you can afford to see a doctor. Lower income folks may have just as many medical issues, but due to affordability may not see a doctor for those issues. So the third factor related to both income level and frequency of doctor visits is affordability.
