Answer (B) is incorrect because the passage is not questioning whether "any conclusion about appropriate therapy" can be drawn. This is far too broad a statement for what the passage does. Rather, all it does is counter the conclusion that older people can develop their memory through bridge. It provides an alternative explanation for the data provided.
Answer (D) is correct because it identifies that in countering the new conclusion, the reasoning simply provides an alternative explanation for the facts provided. We know that bridge players have better memories. At first it was presumed this meant that bridge played a role in that. But, the conclusion challenges this and says maybe bridge is not a cause of the memories, but an effect, because those with better memories simply enjoy it more.
