**Core Concept**
The Net Reproductive Rate (NRR) is a measure of the average number of daughters a woman would have in her lifetime, assuming the current age-specific fertility and mortality rates. The Couple Protection Rate (CPR) is the percentage of couples using some form of modern contraceptive method. The relationship between NRR and CPR is crucial in understanding population growth and fertility trends.

**Why the Correct Answer is Right**
A Net Reproductive Rate (NRR) of 1 implies that, on average, a woman would have exactly one daughter in her lifetime, indicating a stable population. This scenario occurs when the fertility rate equals the mortality rate. A Couple Protection Rate (CPR) of 60% is required to achieve an NRR of 1, as it significantly reduces the fertility rate and leads to a stable population. The CPR of 60% is based on the assumption that 60% of couples are using modern contraceptive methods, thereby reducing the number of children born.

**Why Each Wrong Option is Incorrect**
**Option A:** A CPR of 30% would not be sufficient to achieve an NRR of 1, as it would still result in a high fertility rate and an increasing population.
**Option B:** A CPR of 90% would be excessive to achieve an NRR of 1, as it would lead to a significant reduction in fertility rate and a declining population.
**Option C:** A CPR of 100% is not a realistic or possible scenario, as it would imply that every couple is using modern contraceptive methods, which is not feasible.

**Clinical Pearl / High-Yield Fact**
A CPR of 60% is a critical threshold for achieving a stable population, and it is often used as a target for family planning programs.

**Correct Answer:** C.