Wood is classified as porous if it contains :

Options

A. vessels
B. tracheids
C. companion cells
D. sclereids

Explanation

### Step-by-Step Solution:

1. **Understanding Wood Classification**: Wood is classified into two main categories: porous (hardwood) and non-porous (softwood). This classification is primarily based on the presence of certain types of cells.

2. **Identifying the Types of Wood**:
- **Porous Wood**: Found in angiosperms (flowering plants).
- **Non-Porous Wood**: Found in gymnosperms (non-flowering plants).

3. **Analyzing the Options**:
- **Option 1: Vessels**: These are specialized structures in the xylem of angiosperms that facilitate the transport of water and minerals. They are essential for the plant's ability to absorb large amounts of water, which contributes to the wood being classified as porous.
- **Option 2: Tracheids**: These are elongated cells in the xylem that also transport water but are found in both angiosperms and gymnosperms. They do not contribute to the classification of wood as porous.
- **Option 3: Companion Cells**: These are associated with sieve tubes in the phloem and are involved in the transport of nutrients. They are not related to the classification of wood as porous.
- **Option 4: Sclerids**: These provide structural support and are found in both angiosperms and gymnosperms. They do not contribute to the classification of wood as porous.

4. **Conclusion**: The presence of vessels in wood is what classifies it as porous. Vessels are unique to angiosperms and allow for efficient water transport, which is essential for the growth of flowering plants.

5. **Final Answer**: Wood is classified as porous if it contains **vessels**.