In parallel reasoning, the scope of the conclusion must be very similar. If the original argument has a "usually," then AC's conclusion must have a "most" or "usually" etc. Or if the conclusion is universal, "Bats are nocturnal," then the AC's conclusion must also be universal. (Right?)
In match the flaw, the scope of the conclusion is irrelevant.
I by no means am an expert here. But let me give you my two cents.
(b) is 90% supported by the passage for the reasons you stated. They "must" = they could not have w/o their "must". Also, we know that in MSS questions, often it is the case that only 1 sentence from the passage needs to support the answer choices for it to be correct. This is why I also chose (b)
However, the question is actually very specific in this case. Rather than a general, "which is most strongly supported?" it states the "ARGUMENT is STRUCTURED to lead to which of the following conclusions." There is no one way the structure of the argument leads to (b), (b) is supported with a stretch by one rando statement.
Also, not to be a jerk, but I didn't find one of "Mage of Reasons" points very convincing. I'll rebut them all below quickly just in case it's helpful, but again I'm not trying to be mean, just analytical!
First, it says in the stem they "MUST" have first discovered etc." so we do know they need the knowledge.
Second, the jump for "study natural process, cultivation, and reproduction" to "theoretical knowledge of the principles of plant generation and growth" is one the LSAT makes elsewhere.
Third, if these are the first plants domesticated, then prior to their domestication, they must have just been plants in general. Therefore, a knowledge of plants in general would have been necessary in order domesticate plants. ("Domesticated" is a verb in the stem, not an adjective. Therefore, the stimulus is talking about plants in general).