Answer: brown
Step-by-step explanation:
A natural selection is a process, which can be define as the differential survival and reproduction of the members of the species which exhibit more favourable phenotypic traits over others.
In the given example of beetles. The green colored beetles will camouflage the birds as their color will mimic the color of vegetation. The green color of these beetles will be the benifical phenotypic trait favors the survival and reproduction. The brown colored beetles are the least fit for survival and reproduction as their color make them to be detected by the birds.