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