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

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a material that becomes soft when heated and hardens upon cooling.

2. **Identifying Types of Plastics**: There are two main types of plastics to consider: thermoplastics and thermosetting plastics.
- **Thermoplastics**: These plastics can be softened by heating and will harden when cooled. They can be reshaped multiple times without undergoing any significant chemical change.
- **Thermosetting Plastics**: These plastics do not soften upon heating. Once they are set into a shape, they cannot be remolded or reshaped by heating.

3. **Analyzing the Properties**:
- When we heat thermoplastics, they become soft, allowing them to be molded or shaped. Upon cooling, they harden again.
- In contrast, thermosetting plastics remain hard regardless of heating.

4. **Conclusion**: Based on the properties discussed, the answer to the question is **thermoplastic**.

**Final Answer**: Thermoplastic

---