**Core Concept**
The Net Reproduction Rate (NRR) is a demographic measure that estimates the average number of daughters a woman would have in her lifetime, assuming the prevailing age-specific fertility rates and mortality rates. An NRR of one (NRR=1) indicates that a population is replacing itself from one generation to the next. The Couple Protection Rate (CPR) is the percentage of couples of reproductive age using some form of modern contraceptive method.

**Why the Correct Answer is Right**
To achieve an NRR of one, the CPR must be high enough to reduce the total fertility rate (TFR) to a level where each woman has, on average, only one daughter. This requires a significant reduction in the number of children born per woman. A CPR of 60% or higher is generally considered sufficient to achieve this goal, as it indicates that a substantial majority of couples are using modern contraceptive methods to space or limit their children. This, in turn, leads to a decline in the TFR and an eventual stabilization of the population.

**Why Each Wrong Option is Incorrect**
**Option A:** A CPR of 40% may not be sufficient to achieve an NRR of one, as it implies that only 40% of couples are using modern contraceptive methods, leaving a significant proportion of the population without access to family planning resources.
**Option B:** A CPR of 50% may not be enough to achieve the desired demographic goal, as it indicates that only half of couples are using modern contraceptive methods, leaving a substantial number of women without access to family planning resources.
**Option D:** A CPR of 70% may seem high, but it is not necessary to achieve an NRR of one. In fact, a CPR of 60% has been shown to be sufficient in many contexts.

**Clinical Pearl / High-Yield Fact**
A CPR of 60% or higher is generally considered a critical threshold for achieving an NRR of one, indicating that a substantial majority of couples are using modern contraceptive methods to space or limit their children. This can have significant implications for population growth and development.

✓ **Correct Answer:** C. 60%