Which model describes a population's growth rate?

The model describing a population's growth rate fundamentally hinges on the balance between birth and death rates in a given timeframe. When more individuals are born than die, the population experiences a net increase, leading to growth. This situation reflects a favorable reproductive rate and is a primary driver of population dynamics.

The other choices provide factors that can influence population growth but do not directly define it. Migration (choice B) can affect population size, but it does not inherently describe the growth rate, which is specifically concerned with birth and death. An abundant food supply (choice C) can lead to increased growth rates as it typically supports higher reproduction and survival. However, it does not directly capture the measurement of growth rate, which is predicated on the relationships between births and deaths. Similarly, stable environmental conditions (choice D) can promote a consistent growth rate, but again, they are not the defining elements of growth; they rather provide the context in which growth rates might occur. Thus, the increase in the number of births compared to deaths is the fundamental concept that defines a population's growth rate.