Final answer:
The fertility level at which a population replaces itself is known as the Replacement level, typically estimated at a fertility rate of 2.1 children per woman. This rate accounts for children who may not reach reproductive age, ensuring the population remains stable.
Step-by-step explanation:
The level of fertility at which populations replace themselves is referred to as the d) Replacement level. This is the fertility rate at which women average only enough children by the end of their reproductive years to replace themselves and their partner in the population. The replacement fertility rate is typically a bit above two children per woman—generally estimated at 2.1 in most countries—to account for the fact that some children will not survive to reproductive age or will not have children themselves. A replacement fertility rate less than 2.1 signals a declining population unless augmented by immigration, whereas a rate higher indicates population growth.
Fertility rates contribute to the demographic understanding of a population, alongside mortality rates and net immigration. Fertility rates, measured by the crude birthrate, help demographers to analyze childbearing trends, while mortality rates inform them about death trends in a population. These metrics give insight into the overall growth or decline of a population.