b. density
Km^2 (kilometers squared) is an area of land, so 137 rabbits per km^2 describes the density of rabbits in a given area
Km^2 does have an exponent, but "exponential" has nothing to do with 137 rabbits. A biome is a type of landscape, merely knowing the density of rabbits would not tell the biome. This expression counts the number of rabbits within an area, not just the size of rabbits or size of land area.
