__________ becomes soft when they are heated and harden on cooling.

Options

A. Polythene
B. PVC
C. Bakelite
D. Thermoplastics

Answer

The answer to the question "__________ becomes soft when they are heated and harden on cooling" is **thermoplastics**.

### Step-by-step Solution:

1. **Understanding the Properties of Materials**:
- Materials can be classified based on their behavior when subjected to heat. Some materials soften when heated and harden upon cooling.

2. **Identifying Thermoplastics**:
- Thermoplastics are a type of plastic that becomes soft when heated. This property allows them to be molded into various shapes when they are heated.

3. **Cooling Process**:
- Once the thermoplastic is cooled, it hardens again, retaining the shape it was molded into. This cycle can be repeated multiple times without significant degradation of the material.

4. **Examples of Thermoplastics**:
- Common examples include polyethylene, polyvinyl chloride (PVC), and polystyrene. These materials are widely used in various applications due to their versatility.

5. **Conclusion**:
- Therefore, the correct term to fill in the blank is "thermoplastics," as they exhibit the property of softening upon heating and hardening upon cooling.