The total fertility rate necessary to keep the population constant is approximately equal to:

A) 1.0.
B) 1.5.
C) 2.1.
D) 3.0.

The total fertility rate (TFR) necessary to keep the population constant is called the replacement level fertility. This is the rate at which each woman has, on average, enough children to replace herself and her partner. This value is estimated to be around 2.1 in most developed countries, although it can be slightly higher or lower depending on factors such as mortality rates and sex ratios at birth. Therefore, the best choice is C.