**Core Concept**
The **rapid sand filtration** process is a crucial step in water treatment, removing suspended solids and contaminants from water. This process relies on the size of the **filter media**, typically sand, to effectively trap particles. The size of the particles in the filter media is critical for optimal filtration.

**Why the Correct Answer is Right**
The correct answer is based on the optimal size range for **sand particles** in rapid sand filtration, which is typically between 0.4 and 0.6 millimeters. This size range allows for efficient removal of suspended solids while minimizing head loss and maintaining adequate flow rates. The **hydraulic conductivity** and **porosity** of the sand are directly related to the particle size.

**Why Each Wrong Option is Incorrect**
**Option A:** This size is too small, potentially leading to excessive head loss and decreased filter performance.
**Option B:** This size is too large, resulting in inadequate removal of suspended solids.
**Option D:** This size is also not optimal, as it may not provide sufficient filtration efficiency.

**Clinical Pearl / High-Yield Fact**
Remember, the **effective size** of the sand particles is critical for maintaining the balance between filtration efficiency and flow rate in rapid sand filters.

**Correct Answer:** D. 0.4 - 0.6 mm