Answer:
Option B is the correct answer.
Explanation:
Current number of butterflies in the park = 20 thousand.
Rate of increase of butterfly population = 4% = 0.04
The population of butterfly after 1 year = 20+0.04*20 = 20*1.04
The population of butterfly after 2 years = 20*1.04 + 20*1.04*0.04 = 20*1.04*1.04
The population of butterfly after 3 years = 20*1.04*1.04 + 20*1.04*1.04*0.04 = 20*1.04*1.04*1.04
So, population of butterfly after n years = 20*(1.04*1.04*1.04* .... n times)
Option B is the correct answer.