It should be translation.
Answer:
Why doesn't Dr. Logan feel the bed bug feeding on him? The bed bug injects an anesthetic as it feeds. Bed bugs prefer humans over other hosts. Many blood-feeding parasites carry diseases but bed bugs do not.
Explanation:
I'm not entirely sure what you mean, but a fit animal is one that is able to out-compete with other animals in its ecosystem. It possesses some kind of advantage, which makes it easier to compete with others for food, water, shelter, and mates. Due to these qualities, the animal is obviously able to live for a good deal of time and have a greater chance of producing viable offspring.
Species A and B would be the most similar because there is only one mutation between the two of them located in the first codon.