This is a classic causality argument. Essentially, we're trying to evaluate the hypothesis that a high salt diet causes high blood pressure. More specifically, we're looking for an explanation as to why the hypothesis might not be true.

Think of it as an arrow diagram: high salt --> high blood pressure

The most logical way to weaken this connection is to posit something else that is correlated with a high salt diet, and that is, in fact, the true culprit for high blood pressure.

So we're looking for: something other than salt --> high blood pressure.

Okay. These people in the rain forest community move to the city. They start eating lots of salt. Their blood pressure goes up. One could say that salt is the culprit. But one could also say that people in cities who eat high-salt diets also eat high-fat diets or high-sugar diets, and it's the fat or sugar that's causing problems, not the salt. This is the idea that D is capturing.

C, on the other hand, offers information that has no bearing on the argument. Of course it's possible to eat a low salt-diet in a city. But we're already told, in the question stem, that these folks from the rain forest adopt high-salt diets when they move to the city! We just want to know if it's the salt they're consuming that's causing their newfound medical problems. The notion that they could have consumed less salt if they'd wanted to doesn't shed any light on whether salt is the culprit for their high blood pressure.

Thanks David!
Made me realize how i miss small details in the question stem! Need to work harder

Happy to help! Those little details in the question stem are always the key with Critical Reasoning. I'll typically read the argument twice and consciously scan for those subtle nuances in the language that I'm sure I missed the first time.

Akhil, even if you didn't see that it was already established that after they move they are eating more salt, D would still be better.

What we know is that they move and then experience increases in blood pressure.

So do we care that it's possible to have a low salt diet? Maybe. So if we missed that detail in the argument we might wonder if C were key. It could be a consideration, but the increases in blood pressure do happen for some reason. So maybe it's possible to have a low salt diet, but it's also possible they don't choose that. So C is a little weak.

At the same time, whether after moving to cities they have low salt or high salt diets, and even if we didn't know which it is, D would rock the conclusion. If all we were to know is that they moved and their blood pressures went up, D would make it clear that there are multiple types of dietary changes any of which could underlie the increases, and so the available facts could not be used to "establish salt as the culprit in high blood pressure."

So D is the best in any case.

Working hard, as you mentioned, is good. Yup. At the same time I realized recently that in addition to working hard, I need to cultivate in myself a greater affinity for the idea of working smart. So there's a thought too.