**Core Concept**
The Couple Protection Rate (CPR) is a measure of the percentage of couples using some form of contraception. Achieving a Net Reproduction Rate (NRR) of 1 means that, on average, each couple is replacing themselves with two children, which is the replacement level for a stable population. The CPR is a crucial factor in population control, as it directly influences the NRR.

**Why the Correct Answer is Right**
A CPR of 60% is required to achieve an NRR of 1. This is because, at this level, 60% of couples are using effective contraception, resulting in an average of 2 children per couple. This balance ensures that the population remains stable, with no significant increase or decrease in numbers. The CPR of 60% implies that 40% of couples are not using any form of contraception, which is still a relatively high rate.

**Why Each Wrong Option is Incorrect**
**Option A:** A CPR of 40% is too low to achieve an NRR of 1, as it would result in an average of less than 2 children per couple, leading to a decline in population size.
**Option B:** A CPR of 80% is higher than required, resulting in an average of more than 2 children per couple, leading to an increase in population size.
**Option C:** A CPR of 50% is not sufficient to achieve an NRR of 1, as it would result in an average of less than 2 children per couple, leading to a decline in population size.

**Clinical Pearl / High-Yield Fact**
A CPR of 60% is a critical threshold for population control, and achieving this rate requires a combination of effective family planning education, accessible contraceptive services, and a supportive social environment.

**Correct Answer: C. 80%**