Answer: B. competition
Step-by-step explanation:
A niche can be define as the functional role which an organism plays in an ecosystem to sustain it's life. For example, the type of food the organism eats, or type of habitat chosen.
Generally, when two species exhibit a same niche, competition for resources develop between the two species.
According to the competitive exclusion principle when two or more species compete for the same resource the one which is superior over the other receives the resources and the population of weaker species decline.
In the given situation competition will develop between the two species of mouse as they exhibit same niche and competitive exclusion principle on two mouse population will apply.