Answer:
Option B, 60,000 years ago
Explanation:
It is given that the rate of substitution is one substitution per
years
Thus, starting from the base year
Number of substitution after
years 
Number of substitution after
years 
Number of substitution after
years 
Number of substitution after
years 
Number of substitution after
years 
Number of substitution after
years 
Thus, it is clear that
substitution or changes will occur in
years.
Thus, option B is correct.