**Core Concept**
The Couple Protection Rate (CPR) is a measure used to assess the effectiveness of family planning programs. It is defined as the percentage of couples of reproductive age using any form of contraception. The Net Reproduction Rate (NRR) is a demographic measure that indicates the average number of daughters a woman would have in her lifetime, assuming the current age-specific fertility rates remain constant. An NRR of 1 means that, on average, each woman is having just enough daughters to replace herself.

**Why the Correct Answer is Right**
To achieve an NRR of 1, the CPR should be 100%. This is because, with a CPR of 100%, all couples are using some form of contraception, which would prevent any unintended pregnancies and, consequently, reduce the total fertility rate (TFR) to the replacement level of 2.1 children per woman. This, in turn, would result in an NRR of 1, where the number of daughters born equals the number of mothers.

**Why Each Wrong Option is Incorrect**
**Option A:** A CPR of 50% would result in a higher TFR and an NRR greater than 1, as many couples are not using any form of contraception.
**Option B:** There is no specific CPR value that corresponds to an NRR of 1. The relationship between CPR and NRR is complex and depends on various factors, including the type and effectiveness of contraceptive methods used.
**Option C:** A CPR of 80% would still result in a higher TFR and an NRR greater than 1, as some couples are not using any form of contraception.

**Clinical Pearl / High-Yield Fact**
To remember that a CPR of 100% is required to achieve an NRR of 1, consider the following: "Complete Contraception Prevents Replacement" (CCPR).

**Correct Answer: D. 100%**