"...no survival value in an animal's having an organ..." is grammatically correct, because the apostrophe s on "animal's" indicates a possession, so it is basically saying "there is no survival value when an animal has an organ...". The animal possesses the ability to have such an organ.

I think the stronger reason as to why A is incorrect is the causal reversal, because it more powerfully shows how A is flawed - that it is backwards. I did however turn away from A because of its neutrality (especially in comparison to the strength of D), but I don't think that this is as solid of a reason as its reversed causality. At the end of the day, we are looking for an answer choice that says that personal confidence in their economic state is directly/positively correlated with the confidence a person has in the state of the economy, because then, if someone's confidence in the overall economy decreases, then that person's personal confidence in their own state decreases, and the conclusion would not hold.

MSS questions ask which of the 5 choices are most strongly supported by the stimulus. (Which one is strengthened by the argument)

WSE-S questions ask which of the 5 choices most strongly support the stimulus. (Which choice most strengthens the argument)

They're inverted.

"Lends the most support to" paraphrases "gives-" or "provides the most support to", so it is still a WSE-S question.

Can we also negate "All dogs are friendly" by simply writing "Not all dogs are friendly"?