## **Core Concept**
The net reproductive rate (NRR) is a demographic measure that indicates the number of daughters a woman would have in her lifetime under the prevailing age-specific fertility and mortality rates. An NRR of 1 means that, on average, a woman replaces herself with one daughter, which is essential for sustaining a population over time without growth or decline. The couple protection rate (CPR) refers to the percentage of couples of reproductive age using some form of contraception.

## **Why the Correct Answer is Right**
Achieving an NRR of 1 implies that each woman, on average, has approximately 2.1 children (considering the sex ratio at birth and mortality rates), which effectively replaces herself and her daughter in the population. A higher CPR indicates a larger proportion of couples are using contraception, which generally correlates with lower fertility rates. Research and demographic studies have shown that a CPR of more than 60% is often associated with achieving replacement fertility levels, which corresponds to an NRR of 1. This is because a significant proportion of couples are effectively controlling their family size, leading to lower total fertility rates.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Less than 40% is generally considered too low to impact fertility rates significantly. At such levels, a large proportion of couples are not using contraception, making it difficult to achieve an NRR of 1.
- **Option B:** 40-49% might start to show some effects on fertility rates but is typically still not enough to reliably achieve replacement levels. The impact on NRR would be limited.
- **Option D:** More than 80% is more than what's typically required to achieve an NRR of 1. While it would certainly help in achieving low fertility rates, it's not the minimum threshold.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the couple protection rate (CPR) is a critical indicator of contraceptive prevalence and a useful proxy for understanding fertility trends in a population. For most countries aiming to stabilize their population growth, achieving a CPR of more than 60% is seen as a strategic goal.

## **Correct Answer:** . 60%