The passage first highlights a correlation between people who regularly play the card game bridge and having better short-term memory. The original conclusion from this correlation was thought to be that playing bridge can help older people retain and develop their memory.
However, as with any correlation/causation flaw, it could be that A caused B, B caused A or C caused A and/or B. This is exactly what the passage is saying; the correlation doesn't necessarily mean that playing bridge causes better short-term memory, it could be--in fact--that having better short term memory causes one to want to play bridge more.
So, the correct answer is (D) because the passage, in countering the original conclusion, provided an "alternative hypothesis to explain the data on which the original conclusion was based."
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