If you’re addressing this question under time pressure, (A) should be right because it’s the only answer choice that moves from a belief premise to a fact conclusion.

Its precise structure actually features a few tiny mismatches, although they don’t impact the core “belief doesn’t imply fact” flaw. Here’s (A)’s structure, with the tiny mismatches italicized:

Premise: If a theory is expounded upon, the expounder believes it is [completely] true.
(There’s no implicit premise like “even the dumbest theories that are expounded upon still count as being expounded upon.”)
________
If a theory is expounded upon, it’s [at least a little] true in fact.

If you disliked those mismatches in (A), you’re in good company. But take this question as a data point. Despite those mismatches, (A) is right because its core flaw remains the same as the stimulus’: just because someone believes a theory is true doesn’t mean it’s true in fact.

(B) lacks the stimulus’ fact vs. belief flaw – both claims are fact claims. The premise claims that there is some good to be found, not that people believe that there is some good to be found.

It might be tempting for similarity of subject matter, but that's a common trap in parallel questions.

(C) lacks the stimulus’ fact vs. belief flaw – both claims are fact claims.

As a little metagame sidenote, (C) is also sus because of its subject matter overlap with the stimulus (both talk about leaders bringing about good), which is usually bait for people who don’t realize preserving subject matter is irrelevant in Parallel questions, which are all about structure. That doesn’t make (C) wrong on its own, though. Just sus.

Aside from that, (C) just follows the flawed formal logic pattern called affirming the necessary condition – it takes an arrow that goes one way and tries to make it go the other way. That’s a nono, but it’s a different nono than the one in the stimulus.

Premise: Worthy Leader → Some Good
________
Conclusion: Some Good → Worthy Leader

If you’re coming into (D) with the fact vs. belief flaw firmly in mind, it should actually be kinda tempting – its first premise is pretty facts-vs.-beliefs-y:

People can’t separate their wishes (beliefs) from the truth (facts).

Beyond that passing thematic similarity, though, (D) doesn’t match up very well. Maybe it’s easiest to see by building a model of (D) that would match the stimulus’ structure. I’ll keep (D)’s conclusion and provide a stimulus-mirroring premise:

Matching Premise: People can never believe a theory is completely true.
(D)’s Conclusion: No theory is ever completely true.

If (D) said that, it’d be the right answer. But it doesn’t.

(E) is mainly wrong because it doesn’t feature the stimulus’ facts vs. beliefs flaw. Perhaps the concept of influence is a bit beliefy, but it appears in both the premise and the conclusion anyway, so this argument isn’t jumping from one kind of claim to another.

As a little metagame sidenote, (E) is also sus because of its subject matter overlap with the stimulus (both talk about leaders), which is usually bait for people who don’t realize preserving subject matter is irrelevant in Parallel questions, which are all about structure. That doesn’t make (E) wrong on its own, though. Just sus.