**Core Concept**
Sample registration system (SRS) is a method used to monitor and evaluate the quality of health care services in a community. It involves collecting and analyzing data on births, deaths, and illness in a representative sample of the population.

**Why the Correct Answer is Right**
The SRS is typically conducted once in every 6 months. This frequency allows for the collection of data on a regular basis, enabling the monitoring of trends and patterns in health care services. The data collected is then used to evaluate the effectiveness of health care programs and identify areas for improvement.

**Why Each Wrong Option is Incorrect**
**Option A:** Conducting SRS once a year would result in insufficient data, making it difficult to identify trends and patterns in health care services.
**Option B:** Conducting SRS daily would be impractical and resource-intensive, as it would require a large team of data collectors and analysts.
**Option C:** Conducting SRS once a decade would result in outdated data, making it less useful for evaluating the effectiveness of health care programs.

**Clinical Pearl / High-Yield Fact**
The SRS is a valuable tool for evaluating health care services, but it requires careful planning and execution to ensure the quality and accuracy of the data collected.

**Correct Answer: D. 6 months**