I believe the correct answer from the choices listed above is option D. The term that best describes the expression 137 rabbits per km2 would be density. It is a population density. It is the number of an entity per unit of area, usually quoted per square kilometer or square mile.
Your answer is indeed D. Density, for proof, I took the test recently, and got the asnwer right. Doing so I took a screenshot to share. Have a great day :)
