In a population of beetles, some are green, some are brown, and some are mixed. if the beetles live in vegetation, the colored beetle is "least fit" and least likely to survive in that habitat when birds come along looking for food.

This could go both ways. Green beetles would do well if the vegetation is green, brown beetles would do well if the vegetation is brown, and mixed beetles would do well when the vegetation is a mixture of green and brown.

I think the question is implying that the vegetation is green, so brown beetles would be least likely to survive.

a and b