SomethingSomw
Option B specifies that it takes a month to have 'Influence on Chronic Behavioral Issues", it does not mean that its influence on efficacy of Medicine overall should occur only after a month. Sure its up to interpretation whether the word 'influence' should encompass medication effects aswell or not, but that just means that it should not be a strong Weakener for the Argument.
Option A on the other hand casts a doubt on the Credibility of the research outcome, basically discarding the sole supporter of their conclusion. This seemed like a better answer to me.
is there any specific rule that i should be following here to make a better choice among such options?
(A) is a classic trap choice in a Weaken question.
Notice that it doesn't really change what we know. It just says that judging behavioral patterns is difficult.
OK, great. It's difficult, but according to the passage, they "found that both groups continued to exhibit similar behavioral traits as before." So, it appears that the researchers succeeded in making the difficult judgments.
Simply put, the correct answer to this question can't work by saying that, actually, that was not "found" because finding that would be too difficult.
Admittedly this CR question isn't ideally written because (A) goes at little too far, saying that the "methods ... were not very comprehensive." So, (A) is close to being arguably correct. Still, given how much better (B) is, we can go with (B).
(B) is better because, rather than seeming to conflict with the stated facts, (B) brings up a new fact. Sure, there weren't any improvements, but the participants didn't even have time to learn to meditate effectively. So, it's possible that meditation would have worked well had they been given more time.
So, probably, the fact that the question could be more precisely constructed threw you off a little, but anyway, that's what's going on.