The strength and permeability of sand are related to the moisture content. The correct answer is option 'D' because moisture content plays a crucial role in determining the properties of sand. Let's explore this relationship in detail:

1. Moisture Content:
- Moisture content refers to the amount of water present in the sand. It is expressed as a percentage of the weight of water to the weight of dry sand.
- The presence of moisture in sand affects its strength and permeability. Different moisture contents can result in varying properties of the sand.

2. Strength of Sand:
- Strength refers to the ability of sand to withstand external forces without breaking or deforming.
- The moisture content of sand significantly influences its strength. Dry sand has higher strength compared to moist or wet sand.
- When the sand is dry, the grains are loosely packed, and there is less intergranular contact. This results in higher strength as the force applied is transmitted through individual grains.
- On the other hand, when the sand is moist or wet, the water fills the gaps between the grains, reducing the intergranular contact. This weakens the sand and reduces its strength.

3. Permeability of Sand:
- Permeability refers to the ability of a material to allow the passage of fluids through its pores or void spaces.
- The moisture content of sand also affects its permeability. Dry sand has higher permeability compared to moist or wet sand.
- Dry sand has well-defined pore spaces between the grains, allowing fluids to flow easily. This results in higher permeability as there is less resistance to the flow of fluids.
- In contrast, when the sand is moist or wet, the water fills the pore spaces, reducing the permeability. The flow of fluids through the sand becomes restricted due to the presence of water.

In conclusion, the moisture content of sand plays a significant role in determining its strength and permeability. Dry sand has higher strength and permeability compared to moist or wet sand. Therefore, option 'D' - moisture content is the correct answer as it directly influences the strength and permeability of sand.