The correct option for the question regarding which type of soil has the highest water retention is C. clay and humus, but poor in sand. Detailed Explanation:

1. Understanding Soil Composition:
2. Soil is composed of various particles, primarily sand, silt, and clay, along with organic matter (humus). Each of these components has different properties that affect water retention.
3. Sand: Sand particles are large and have a gritty texture. They create large spaces between particles, allowing water to drain quickly. Therefore, sandy soils do not retain water well.
4. Clay: Clay particles are very small and have a flat shape, which allows them to pack closely together. This close packing creates small pores that can hold water effectively. Clay soils tend to retain water for longer periods.
5. Humus: Humus is decomposed organic matter that improves soil structure and increases its ability to retain moisture. It can hold water like a sponge and also provides nutrients to plants.
6. Why Option C is Correct:
7. Clay and Humus: When soil is rich in both clay and humus, it has a high capacity for water retention. The clay holds water due to its small particle size and high surface area, while humus enhances this ability by providing additional moisture retention and nutrient availability.
8. Poor in Sand: The absence of sand in this option means that there are no large particles to create drainage pathways, which would otherwise allow water to escape quickly. Thus, the combination of clay and humus maximizes water retention.
9. Why Other Options are Incorrect:
10. Option A (sand, poor in humus and devoid of clay): This option is incorrect because sandy soils do not retain water well. The lack of humus further reduces the soil's ability to hold moisture, making it the least effective option for water retention.
11. Option B (clay and sand, but poor in humus): While clay does retain water, the presence of sand in this mixture would allow for quicker drainage, reducing overall water retention. The lack of humus also means that the soil would not benefit from the additional moisture-holding capacity that humus provides.
12. Option D (clay, poor in humus and devoid of sand): Although clay retains water well, the absence of humus means that the soil may not retain as much moisture as it could if humus were present. Humus is crucial for enhancing water retention, so this option is weaker than option C.

Summary of Key Points:

• Water Retention: Clay and humus together create optimal conditions for water retention due to their physical properties.
• Sand's Role: Sand decreases water retention because of its large particle size and quick drainage capabilities.
• Importance of Humus: Humus significantly enhances the soil's ability to hold moisture and provides essential nutrients.
• Best Combination: The best soil for water retention is one that is rich in clay and humus while being low in sand.

By understanding these concepts, students can better appreciate the relationships between soil composition and water retention, which is crucial for agriculture, gardening, and environmental science.