D. fingernails
Skin, hair, and vertebrae are all the outer layer of the body and protect the body. The fingernails aren't part of that.
competition
source: my biology teacher
This definition above refers to the geographic isolation.
It means exactly what it says in the definition - one species is left behind from the rest of the population, and it stays in that particular location, evolving and changing from the species and population it was once a part of.
