medullary rays are extra prominent in

Options

A. monocot stem
B. dicot stem
C. young dicot root
D. old dicot root

Answer

### Step-by-Step Solution:

1. **Understand the Concept of Medullary Rays**:
- Medullary rays are line-like structures found in the plant's stem and roots. They are composed of parenchymatous cells and are located between vascular bundles. Their visibility is associated with secondary growth in plants.

2. **Analyze the Options Provided**:
- The options given are monocot stem, dicot stem, young dicot root, and old dicot root.

3. **Evaluate Each Option**:
- **Monocot Stem**: In monocots, medullary rays are generally absent. Therefore, this option can be eliminated.
- **Dicot Stem**: Dicot stems do have medullary rays, but they are not as prominent as in older structures.
- **Young Dicot Root**: Young roots do not exhibit secondary growth, so medullary rays would be absent in this case.
- **Old Dicot Root**: In old dicot roots, secondary growth occurs, which leads to the development of prominent medullary rays.

4. **Conclusion**:
- Based on the analysis, the option that indicates where medullary rays are extra prominent is the **Old Dicot Root**. This is due to the presence of secondary growth, which enhances the visibility of medullary rays.

### Final Answer:
**Old Dicot Root**