**Core Concept**
The Sample Registration System (SRS) is a national system designed to provide reliable estimates of fertility and mortality rates in India. It involves a continuous enumeration of births and deaths by an enumerator and a supervisor, who visit households regularly to collect data. The SRS is a crucial tool for policymakers and researchers to understand demographic trends and make informed decisions.

**Why the Correct Answer is Right**
The SRS is a dual-record system that combines the continuous enumeration of births and deaths by an enumerator with the periodic enumeration of births and deaths by the supervisor. This dual-record system helps to eliminate errors and biases that can occur in single-record systems. The SRS also uses a sampling methodology to cover a representative sample of the population, which enables the generation of reliable estimates of fertility and mortality rates. The system is designed to provide annual estimates of fertility and mortality rates at the national and state levels.

**Why Each Wrong Option is Incorrect**
**Option A:** The Civil Registration System (CRS) is a system that records births and deaths at the time of occurrence, but it is not a national system that provides reliable estimates of fertility and mortality rates. While CRS is essential for vital statistics, it often suffers from underreporting and errors.

**Option B:** The Census is a comprehensive survey that provides data on population characteristics, but it is conducted at a decennial interval and does not provide annual estimates of fertility and mortality rates. The Census is a valuable tool for demographic analysis, but it is not a continuous system that provides timely data.

**Option C:** Ad-hoc surveys are conducted for specific purposes and may not provide reliable estimates of fertility and mortality rates. These surveys are often limited in scope and may not cover the entire population, which can lead to biases and errors.

**Clinical Pearl / High-Yield Fact**
The Sample Registration System is a unique system that combines the strengths of continuous enumeration and sampling methodology to provide reliable estimates of fertility and mortality rates. This system is essential for policymakers and researchers to understand demographic trends and make informed decisions.

**Correct Answer Line**
✓ Correct Answer: D. Sample registration system