To play the LSAT game of "analyze the REASONING / ARGUMENT",
we play by a very specific set of rules:
accept the truth of the Evidence, but figure out why that still doesn't justify the Conclusion.
The evidence:
Arnot's argument depended on a dubious assumption.
The conclusion:
Arnot's conclusion was false.
So the thinking game is
"GIVEN THAT Arnot's argument depended on a dubious assumption,
HOW COULD I STILL ARGUE Arnot's conclusion was true?"
(A) describes how we would win that conversation. We can say, "A conclusion can be a correct claim, even if the argument made on its behalf was faulty."
Here's an example:
CONC: There are fewer atoms of of oxygen in water than there are atoms of hydrogen.
EVID: The molecular formula for water is H3O.
My evidence is wrong, but my conclusion is still correct.
If the thinking game is
"GIVEN THAT Arnot's argument depended on a dubious assumption,
HOW COULD I STILL ARGUE Arnot's conclusion was true?"
then the line of objection you're making is
"Nuh-uh. His argument didn't depend on a dubious assumption."
But we're never allowed to make the objection of
"Nuh-uh. Your premise is wrong."
We're only ever making the objection of
"Even if your premise is valid, I still don't have to arrive at your conclusion."
Hope this helps.