Zero population growth is achieved when ____.

A) a population reaches the carrying capacity of the environment
B) the number of births is balanced by the number of deaths
C) births exceed deaths
D) immigration exceeds emigration
E) migration is prevented

Zero population growth occurs when the number of births is equal to the number of deaths. This means that the population is neither increasing nor decreasing, but is remaining stable. Option A is incorrect because carrying capacity is the maximum number of individuals that can be supported by an environment and does not necessarily lead to zero population growth. Option C is incorrect because when births exceed deaths, the population is increasing. Option D is incorrect because when immigration exceeds emigration, the population is also increasing. Option E is incorrect because migration, whether prevented or not, does not directly affect the population growth rate.