I would say the conclusion is the following:
"If Diet soda was substituted for regular soda, it would reduce our chances of getting cancer"

Thought. "What makes this statement weaker?" - A fact about diet soda being unhealthy.

Therefore, I would look at A and D.

A.) if this had pancreatice cancer instead of other cancers would it be correct?
D.) if this said linked to a higher risk of pancreatic cancer would it be correct?

E.) Isn't this weakening because the more sugar drinks you have it doesn't increase the rate of pancreatic cancer?

I understand that B offers an alternative explanation. I also see that it is a
confounding variable, but its hard to match that up with the conclusion.

What should my thought process be on weakening problems? Especially this where it leads you to want to weaken diet soda. Thank you.

It's a risky proposition going outside of what's printed and changing the answers to be "also correct."

In this case, you're right that A goes out of scope by mentioning the other types of cancers. However, it also doesn't address the implied causality in the argument. The author seems to conclude that consuming sugar CAUSES cancer. However, the evidence simply says that those people who are more likely to consume sugar are also more likely to have this specific kind of cancer. Fixing the type of cancer in A doesn't completely fix the argument, but it plugs the biggest hole.

D. Doesn't weaken the argument - just addresses another characteristic of these people. In fact, this characteristic might, given the right evidence added to it, strengthen the argument...

E. Again - we have to go back to "sugar causes cancer" as the bad logic here. The evidence is "tendency to consume sugar correlates to tendency to contract this specific cancer." E doesn't weaken this.

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