For this answer choice to be relevant, we would need to know whether exercising regularly is connected to arthritis--but we aren't told that there is any relationship. Even if we knew exercise was related to arthritis, we would need to know the difference in exercise rates between people who drink decaffeinated coffee and people who drink regular coffee, not the difference in likelihood of drinking decaffeinated coffee among people who exercise frequently versus people who don't. This answer choice is irrelevant.

Even if, in general, people who drink decaffeinated coffee tend to drink coffee less often than those who drink regular coffee, we know from the stimulus that we're only focused on people who actually drink coffee, whether decaf or regular, at the same frequency: three cups a day. So this answer choice brings in a trend that is irrelevant.

This is very relevant to evaluating the argument. If this is true, then the different rates in arthritis might not be because decaffeinated coffee contains something that causes arthritis, but instead that caffeinated coffee is staving off arthritis. This would weaken the author’s argument. If caffeine doesn’t slow degeneration, the argument remains intact.

The study is focused on two groups of people who each drink three cups per day. The hypothesis about the different rates of arthritis between these groups could still be true even if most people actually drink more or less than three cups a day. They would just be running a greater or smaller risk of arthritis accordingly.

Irrelevant. We’re not interested in people who already have arthritis, and how often they drink coffee. We’re interested in the connection between drinking coffee and eventually developing arthritis.