Good question! Your thinking is exactly right on this. You caught the flaw--the false contrapositive--and you found it in (B).
And I'm including in my praise your note that "many" and "often" throw things a bit--you need to be wary of these terms. Rather than give you an absolute rule about whether you can diagram conditional logic statements with "many" and similar terms, I advise doing what you did here. You noted it, kept it in mind, and still found a flaw in the reasoning by working through a conditional-logic interpretation of it.
We of course need to be careful, because when we write:
Dogs --> Bark
It means if it's a dog, then it barks, right? ALL dogs bark. If it becomes "many" dogs bark, the meaning is different. But we can often take care of this in matching questions like this one by throwing "Many" in front of "dogs" and looking for a match. That works here, as you found.
(A) doesn't have a flaw.
(C) just gives us one relationship between two phenomena then calls one of them important.
(D) may be tempting to some. But we can think of it as:
Allow vivid conceptualization --> Using visual aids effective
If we didn't use visual aids, teaching math might be harder.
It's just throwing out an example that fits the statement. Also, "teaching math might sometimes be harder" is a weaker conclusion than what we have in the stimulus.
(E) Successful painter --> understand rules, therefore, if you understand the rules, you'll be successful. This is a mistaken contrapositive of another kind: it flips without negating. We're looking for one that negates without flipping.
#officialexplanation