**Core Concept**
The Sample Registration System (SRS) is a dual-record system that involves continuous enumeration of births and deaths by an enumerator, and a survey of a sample of households by an investigator every six months. This system provides reliable estimates of fertility and mortality rates at the national and sub-national levels.

**Why the Correct Answer is Right**
The SRS is a dual-record system that involves continuous enumeration of births and deaths by an enumerator, and a survey of a sample of households by an investigator every six months. This dual-record system helps to validate the data collected and provides more accurate estimates of fertility and mortality rates. The SRS also uses a large sample size, which helps to reduce the standard error and provides more reliable estimates.

**Why Each Wrong Option is Incorrect**
**Option A:** The SRS is a continuous survey that provides data on fertility and mortality rates. It is not a one-time survey, so this option is incorrect.
**Option B:** The SRS uses a sample size of 1,000 households in each district. However, the actual sample size may vary depending on the district and the population size, so this option is an oversimplification and incorrect.
**Option C:** The SRS provides reliable estimates of fertility and mortality rates at the national and sub-national levels. It is a widely used and respected source of demographic data, making this option incorrect as it is actually a true statement about the SRS.

**Clinical Pearl / High-Yield Fact**
The SRS is a valuable resource for demographers, researchers, and policymakers, providing reliable estimates of fertility and mortality rates that can inform public health policy and programs.

**Correct Answer: D.**