**Core Concept**
The Sample Registration System (SRS) is a national-level system used to estimate vital statistics, including fertility and mortality rates, in India. It is a complex system that combines the efforts of the Census of India and the Office of the Registrar General of India to provide reliable and accurate data.

**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 an investigator. The enumerator records the events on a monthly basis, while the investigator conducts a quarterly survey to verify the events recorded by the enumerator. This dual-record system helps to reduce errors and ensures the accuracy of the data collected. The SRS provides annual estimates of fertility and mortality rates at the national and state levels, making it a valuable tool for policymakers and researchers.

**Why Each Wrong Option is Incorrect**
**Option A:** Civil registration system is a system of recording vital events, but it does not provide reliable estimates of fertility and mortality rates, especially in rural areas where registration is often incomplete.

**Option B:** Census provides population data, but it does not provide reliable estimates of fertility and mortality rates, as it is conducted only once every decade.

**Option C:** Adhoc survey provides limited data on specific topics, but it is not a systematic approach to estimating fertility and mortality rates.

**Clinical Pearl / High-Yield Fact**
The Sample Registration System is a unique system that provides reliable estimates of fertility and mortality rates in India. It is a valuable tool for policymakers and researchers, and its data are used to inform public health policies and programs.

✓ Correct Answer: D. Sample registration system