We need to find the minimum of all in a 3 set venn diagram which can be calculated using a shortcut as follows
Take the difference from 100 for each of the groups
Pop= 100-100 =0
Sum of all the differences = 75.
Take the difference from 100 again to get the minimum = 100-75= 25%
Answer= 25% of 60000 = 15000. Option (a)