The total fertility rate in an economy is defined as:

The Total Fertility Rate (TFR) is defined as the average number of children a woman is expected to bear during her reproductive years, assuming current age-specific fertility rates remain constant.

In simpler terms, it estimates the average number of live births a woman would have by the end of her child-bearing age.

For example, India's TFR has significantly decreased from approximately 6.2 children per woman in 1950 to under 2 children per woman in 2021.

A TFR of about 2.1 children per woman is known as Replacement-level fertility. A TFR lower than 2.1 suggests that a population isn't producing enough children to replace itself, potentially leading to population decline.

Hence option (d) is the correct answer.