Answer: A. The beetle population moved to a greener habitat.
Explanation: this is an example of one of the forces of microevolution called natural selection as Charles Darwin proposed which states that organisms that survive are those who adapt better to their environment. In this case, the decrease in black beetles population respect to green beetles could be caused due to a selection in a greener environment in which successful beetles were those who could hide better to avoid predators (green ones) and reproduce to increase their proportion in the whole population.
Also, it is possible to reject B and c option because the number of insects remain stable so it's not a resource problem or and extinction and D is rejected because green color isn't new after some generations, this characteristic already existed.