Some scientists believe that sugar causes a specific type of pancreati

One of the most important things in CR - you have to strengthen/weaken the conclusion of the stimulus. Just the conclusion.

So first thing to look for is the conclusion. It is pretty straight forward here, isn't it?
It starts with 'Therefore' and fits the description of conclusion perfectly. It is the conclusion of the stimulus then.

Focus on the conclusion: if we were to substitute diet drinks for regular soda pop, it would considerably reduce our chances of getting pancreatic cancer

What is it trying to tell you? It is saying that if you reduce your sugar intake, your chances of getting pancreatic cancer reduces.

I think the only confusion is between (B) and (C).
(B) is correct because it gives you an alternative cause of pancreatic cancer - no regular exercise. Since people who consume sugar drinks are also the ones who exercise little, the ones who consume diet drinks could be more active and hence, there could be an alternative explanation. Hence the conclusion that reduction in sugar will reduce your chances of getting pancreatic cancer is weakened.

(C) is a sneaky but incorrect option. It kind of strengthens the link between sugar and cancer, doesn't it? People who drink regular pop also consume more sugar in other things. So probably, people who consume diet drinks take less sugar in other things too and hence, their risk of cancer is lower. Don't forget the intent of the conclusion - it is establishing a relation between sugar and cancer, not between soda pop and cancer. The whole stimulus is based on the amount of sugar.