Answer
C, there is an SNP affecting a gene that is decreasingly common in those over 50.
Work Step by Step
A. Is unlikely because a SNP mutation would be a steady degradation over time and would not just occur suddenly at a certain age. As such, it would be noticed in more participants, not just those over 50.
B. Is unlikely because genetic variations (evolution/mutation if you'd prefer) takes more than 50 years to form. For a genetic variation to become especially prominent takes hundreds of years.
C. Is the answer as an SNP could effect a gene and cause it to occur less frequently.
