First, we must know the concept of fitness, which is the ability of an organism to survive and produce offspring in its environment.
In the population of rocket pocket mice, we can find dark fur and light fur.
In this case, the dark fur may present an advantage in rocky landscapes, something that in fact occurs in populations of these mice that inhabit volcanic rocky terrain. While the dark mice mimic, the light-furred mice are easier to hunt.
Therefore, having a dark coat would be an example of fitness in rocky or dark-colored landscapes. Hence, the answer is B (The dark mice are better at hiding)