The replacement level fertility rate refers to the average number of children that a woman needs to have in her lifetime in order to replace herself and her partner in the population. It is the rate at which the population remains stable over time, with births and deaths balancing each other out.

The correct answer to this question is option 'B', which is 2.1. This means that, on average, each woman needs to have 2.1 children in order to maintain a stable population size.

Explanation:
There are several factors that contribute to the replacement level fertility rate:

1. Mortality Rate: The replacement level fertility rate takes into account the mortality rate of a population. It assumes that some children will not survive to reproductive age, and thus, a slightly higher fertility rate is required to compensate for these deaths.

2. Gender Ratio: The replacement level fertility rate also considers the gender ratio in a population. Since only women can bear children, the rate needs to account for the fact that not every person in the population is capable of reproduction.

3. Desired Family Size: The replacement level fertility rate is influenced by the desired family size of individuals in a population. If the desired family size is lower than the replacement level fertility rate, the population will decline over time. Conversely, if the desired family size is higher, the population will continue to grow.

4. Population Age Structure: The replacement level fertility rate is also affected by the age structure of a population. In countries with a large proportion of elderly individuals, a higher fertility rate may be required to maintain a stable population size.

In conclusion, the replacement level fertility rate is the average number of children that a woman needs to have in order to maintain a stable population size. The correct answer to this question is option 'B', which is 2.1. This rate takes into account factors such as mortality rate, gender ratio, desired family size, and population age structure.