**Core Concept**
A rapid sand filter is a type of water filtration system used in municipal water treatment plants. It is designed to remove particulate matter and contaminants from water by passing it through a bed of sand. The size of the sand particles is critical in determining the effectiveness of the filtration process.

**Why the Correct Answer is Right**
The size of the sand particles in a rapid sand filter is typically in the range of 0.4 to 0.7 mm. This size range allows for efficient removal of particulate matter, including bacteria, viruses, and protozoa, while also preventing the sand from becoming too compacted and reducing the flow rate of the water. The sand size is critical in maintaining the desired flow rate and ensuring that the filter can effectively remove contaminants.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.1 mm is too small for rapid sand filtration, as it would result in excessive sand compaction and reduced flow rates.
**Option B:** 0.2 mm is also too small, as it would not provide sufficient removal of particulate matter.
**Option C:** 0.5 mm is too large, as it would not allow for efficient removal of smaller particles and contaminants.

**Clinical Pearl / High-Yield Fact**
When designing a rapid sand filtration system, it is essential to select sand particles with a size range of 0.4 to 0.7 mm to ensure effective removal of contaminants while maintaining optimal flow rates.

**✓ Correct Answer: D. 0.4 to 0.7 mm**