Hi ashpine17!
You ask,
for c it says main conclusion but i accidently read it as sub conclusion; if it had been the sub conclusion would this have been correct? is "supposed experts" a problem here?
To the second question, I can see how one might interpret the stimulus as teeing up an appeal to expertise as the flaw in the reasoning. The second sentence of the stimulus indicates that a set of economists won a prestigious prize and also all have the Acme retirement plan (and this is in an advertisement). Perhaps economists or the specific subset of them mentioned are comparatively likely to be experts on what the best retirement options are. But it's also worth noting that the stimulus doesn't anywhere connect these dots in terms of how their expertise qualifies them on retirement plans. So a problematic appeal to expertise is reasonably a flaw that one might prephrase after reading this stimulus.
To the first question, even though that type of flaw is mentioned in (C), this answer is still problematic. I don't see how changing "main conclusion" to "subconclusion" would make it correct. Answer choice (C) states, "It appeals to the fact that supposed experts
have endorsed the argument's main conclusion, rather than appealing to direct evidence for that conclusion." The italicized language seems problematic. The conclusion in the stimulus is: "it is probably a good plan for anyone with retirement needs similar to theirs." The advertisement doesn't say that the mentioned economists endorsed that the retirement plans they have are a good plan for people with similar needs as theirs.
Instead, answer choice (D) mentions a different flaw and is accurately tracking what is in the stimulus. Answer choice (D) states, "It takes for granted that some winners of the Economic Merit Prize have deliberately selected the Acme retirement plan, rather than having had it chosen for them by their employers." The advertisement does too much with the fact that the mentioned subset of economists all have the same retirement plan. You mention,
even if their employer did choose the plan for them, it doesn't necessarily follow that they think the plan is bad. for all we know, they think the plan is good. that' why i have big issues iwth D
Yes, based on the language of the advertisement, we don't know one way or another whether the economists like or don't like the plans they have. This is why the advertisement does too much with the fact that they --have-- this particular plan--it seems like the advertisement assumes that their having the plan implies that they endorse the plan. That's not necessarily the case, they might dislike the plan, or they might be neutral on the plan and have it because their employer selected it for them.
" makes it sound as the argument is not acknowledging that some of them may have chose it themselves which to me sounds more like supporting the conclusion rather than finding a flaw. 
