Tanning of leather is

Options

A. colouring of leather by chemicals
B. drying process to make the leather hard
C. polishing of leather to make it look attractive
D. hardening of leather by coagulation

Explanation

### Step-by-Step Solution:

1. **Understanding Tanning of Leather**:
- Tanning is a process that transforms animal hides into leather, making it more durable and resistant to decomposition.

2. **Mechanism of Tanning**:
- During tanning, the animal hides contain positively charged colloidal particles, while the tanning agents (often derived from plants) have negatively charged colloidal particles.

3. **Coagulation Process**:
- When the animal hides are soaked in the tanning solution, the negatively charged tanning agents interact with the positively charged particles in the hides. This leads to a process known as mutual coagulation.

4. **Result of Coagulation**:
- The mutual coagulation causes the leather to harden, which is the primary goal of the tanning process. This hardening makes the leather more durable and suitable for various uses.

5. **Identifying the Correct Option**:
- Now, let's analyze the provided options:
- **Option 1**: Colouring of leather by chemicals - Incorrect, as tanning is not primarily about coloring.
- **Option 2**: Drying process to make leather hard - Incorrect, as tanning involves more than just drying.
- **Option 3**: Polishing of leather to make it look attractive - Incorrect, as polishing is not part of the tanning process.
- **Option 4**: Hardening of leather by coagulation - Correct, as this accurately describes the tanning process.

6. **Conclusion**:
- Therefore, the correct answer is that tanning of leather is the hardening of leather by coagulation.