I'm in a similar position regarding this. I remember from my time studying for the LSAT that one of the points to weakening reasoning is to focus on weakening the conclusion, not the premises.

The argument concludes there is a mistake.
The facts are that it is 1) on a women targeted show 2) majority of frequent users are men

The question we need to ask is "why is there not a mistake?"
A) If the purchasers of products are not the end users, it allows us to break the apparent logical discrepancy in play. There is not a mistake if this is true because it means that the end users do not have to watch the TV show. This choice doesn't mean that it is impossible there is a mistake, but it certainly provides us with an alternative situation in which a purchaser may have different behaviors than the end user group, thus explaining the targeting variance.
B) Many women occasionally use the tools. Here we are generous to the network in assuming that they will target their ads to the primary source of the sale and that an occasional user does not qualify in that group. If we look at the group of occasional users, it appears they are out of the scope. Choices that are out of scope do not tend to weaken. Furthermore, this choice really focuses on the premises by questioning the demographics - it tries to make us wonder whether women in fact predominant or significant users of tools. However, we know what we need from the premises: the majority of frequent users (implied marketing targets) are men.
D) This weakens, although it is very limited in its ability to weaken. It would be stronger if the question placed more absolutes, "only women" "only frequent users" "no men" etc. We know from it that some men who frequently use the tools occasionally watch the cooking shows. Does this necessarily mean that the network made a mistake? No - they could have targeted the very small subset of the population, but if we give them benefit by assuming that they target their ads to the primary source of the sale, then it would appear a mistake remains.

IMO B

Yet I think I understand why it is wrong?

The argument states that the company had already made the analysis and then they found out men are the ultimate USERS (NOT THE BUYERS IT KICKS OUT D) of these tools. So we have to weaken the argument by prove that the company advertised their product on cooking program, deliberately. Besides, answer B has the same flawed approach the company adversited its products to increase its sales. Many women using these tools does not mean they buy them too.

A is the only option left , although it is not so clear for a non-native test taker like me.

I think we all understand that A and D are the only contenders in comparision to other choices because -
B) word occasionally speaks for itself - thus fails to suggest strongly that women are the main targetted audience.This could have weakened the arguement.
c) Does not matter whether AD Agency aired in other programs earlier or not.
e) The AD Agency has reputation for conducting high quality demographic research.- so this may hint towards actual target segment comprised of women
which is against the fact in the para.

I wont bank on D as The target audience cannot be limited to such a small segment saying 'Some men including some who use...'.

So, ultimately even by choice of elimination you are left with A which strongly weakens the entire arguement of non-catering to worthy audience for the product by saying - The purchasers of products given as gifts are not the ultimate users of those products - so women audience would gift to men (their husband or servant or housekeeper..whomever to use these tools for home improvement..
Thus, A looks most justifying here.