tough problem.

remember that you must stay within the scope of the argument. you should develop a keen sense for what is, and what isn't, relevant to the issue(s) at hand.
in this problem, the argument is concerned solely with the retail market; non-retail business, such as the direct contracting mentioned in choice (a), is irrelevant to the discussion.

here's the basic skeleton of the argument:
* someone has attributed small firms' struggling in the equipment retail business to the entry of superstores into that retail market.
* this is flawed because the superstores control a low % of the market.

your goal here is to weaken the argument, which, as always, is done by undermining assumptions.
it's actually fairly straightforward to pick up on the key assumption in the argument here, because a connection is made, without any explicit justification, between (a) the superstores' low market share in the retail business and (b) their purported inability to affect the bottom line of the other firms in the business.
therefore, the argument (the original argument, which the narrator says is flawed) makes the following assumption: stores with a small market share can't possibly affect the other firms in the market.

therefore, we need to find an answer choice that gives a way in which stores can affect the other firms in the market even if those stores have a small market share.

choice (b) does this: the stores' heavy advertising is independent of their small market share, so this choice gives a way in which the superstores, despite a small market share, can have a significant impact on the rest of the market.

to reiterate, choice (a) is irrelevant, because the argument is concerned solely with the retail market, and so any consideration of the non-retail market is outside the scope of the argument.