Which of these is a part of soil that is most helpful to the growth of plants?

Options

A. Humus
B. Sand
C. Clay
D. Small porous rocks

Explanation

**Step-by-Step Solution:**

1. **Understanding Soil Composition**: Soil is made up of various components, including minerals, organic matter, air, and water. It is important to identify which part of the soil contributes most to plant growth.

2. **Identifying Soil Components**: The question provides several options: humus, sand, clay, and small porous rocks. We need to evaluate each of these components to determine their role in plant growth.

3. **Evaluating Each Option**:
- **Sand**: While sand is a component of soil, it primarily provides drainage and does not retain nutrients well.
- **Clay**: Clay holds water but can become compacted, making it difficult for roots to grow. It also does not provide many nutrients.
- **Small Porous Rocks**: These do not contribute significantly to nutrient content and primarily aid in drainage.
- **Humus**: Humus is the organic matter formed from the decomposition of dead plants and animals. It enriches the soil with nutrients and improves its structure, making it vital for plant growth.

4. **Conclusion**: Among the options given, humus is the most beneficial part of soil for plant growth because it provides essential nutrients and improves soil fertility.

5. **Final Answer**: The part of soil that is most helpful to the growth of plants is **humus**.

---