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In the first LR section of this PT, I noticed a strange sufficient assumption correct answer that I didn't encounter before.

The argument in the stimulus is essentially:
conclusion: A -> D
since anything that B -> C
and C -> D
and IF data is correct, A -> B.

So IF data is correct, A->B->C->D

The stem says "the conclusion drawn above follows logically if which one of the following is assumed"

C) and D) were attractive answer choices to me, because C) essentially restated B->C, and D) stated the data were correct.

Because I thought the argument was valid, I thought the assumption could entail just reiterating a premise in a stronger way, because the argument is not flawed by saying "if A->B, A->D". However, the stem states the conclusion follows if... and hence it's only true that "A->D" if the data were correct.

Correct me if I'm wrong, but I don't remember seeing another assumption question where the correct answer entailed confirming that a conditional was true in the stimulus. If the conclusion's force was 100%, but a premise is a conditional that only could be true, then I guess the conclusion is invalid? Usually in assumption questions, I would quickly eliminate ACs that deal with only the premises, not the conclusion, but here the right answer only dealt with one of the premises. In assumption ACs, I also look for connecting terms that show up in premises but not the conclusion and as a general rule, they are correct, which is also why D) seemed like a surprising style of answer choice.

Has this happened in other assumption questions, and is it likely that it will happen on a modern one?

tl;dr If an answer choice in an assumption question is completely equivalent to a stated fact in the stimulus, then can it always be safely eliminated by not being an "assumption"? Also, what other question types might have confirming a conditional premise be the correct answer choice?

thank you

You're right about answer choice C being a reiteration of a premise, and that being inadequate. Repeating a premise does not strengthen an argument, and has never been correct on an assumption question.

About D, I'm not sure about your analysis. The argument in the stimulus is definitely invalid. We have a premise that states that a key component of the conclusion is valid IF the study is correct. So essentially,

P: B --> C --> D
P: Study Correct --> (A-->B)
--------------------------------
C: A --> D

Assumption: Study Correct

That assumption is a bit unusual, it's just a trigger for one of the conditional statements. That's not confirming that a conditional relationship was true, rather just asserting that the sufficient condition had been met, allowing us to infer that the necessary condition had been met. It's a weird answer choice for a sufficient assumption question, but not terribly rare for the test in general. It's essentially just doing modus ponens, which is less commonly tested than the more complicated contrapositive structure (modus tollens).

Ah you're explanation cleared up what I was hung up on. I was focused on the conditional statement being "true" rather than thinking about how the sufficient is being "triggered" or satisfied. This will be useful for if I ever encounter this again. Thank you!