When looking to strengthen a plan or proposal, the most important assumption to confirm is often, "it will work." In this case, the plan is to set up farmstands to diversify, so that the acreage does not go solely to the production of apples. In order for that plan to work, there has to be some reason to believe that it will make money, that people will buy something other than apples.

Choice (B) provides the best answer, confirming that there is a market in the region for things other than apples. Notice that (C) and (D) are decent answers. Both are important parts of the plan, but neither provide as much evidence that the plan will work as (B) will. In (C), we don't know that the passersby would buy anything, and in (D), we don't know that anything would be purchased, even if costs are low.