**Timber as a Natural Material**

Timber is a natural material that is widely used in construction and various other applications. It refers to wood that has been processed and prepared for use in building structures, furniture, and other wooden products. Timber comes from trees, which are living organisms made up of different components.

Among the given options, the correct answer is option 'A' - Lignin with fibres. Let's understand why this is the correct answer.

**Explanation:**

1. **Lignin with Fibres:**
- Lignin is a complex organic polymer that is present in the cell walls of trees and other woody plants.
- It provides strength and rigidity to the timber by binding the cellulose fibers together.
- The fibers, composed mainly of cellulose, are long, slender, and arranged parallel to each other.
- This arrangement gives timber its characteristic strength and ability to resist bending, compression, and tension forces.

2. **Medullary Rays:**
- Medullary rays are another component of timber, but they are not the correct answer in this case.
- Medullary rays are radial structures in wood that allow the transport of nutrients and water between the tree's core (pith) and the outer layers.
- They are responsible for the radial texture often visible in timber, but they do not provide the primary strength and structure.

3. **Heartwood:**
- Heartwood is the non-living central part of the tree trunk.
- It is darker in color and provides structural support to the tree.
- However, heartwood alone does not represent the entire composition of timber.

4. **Sapwood:**
- Sapwood is the living outer part of the tree trunk.
- It transports water and nutrients from the roots to the leaves.
- Sapwood is lighter in color compared to heartwood and is generally less dense.
- While sapwood is a component of timber, it is not the primary constituent responsible for its strength.

Therefore, the correct answer is option 'A' - Lignin with fibres.