Which of the following is not used as a rocket fuel?

Options

A. Synthetic rubber
B. Liquid hydrogen
C. Paraffin
D. Liquid nitrogen

Answer

To determine which of the following options is not used as a rocket fuel, we will analyze each option based on their properties and suitability for rocket propulsion.

### Step-by-Step Solution:

1. **Understanding Rocket Fuel Requirements**:
- Rocket fuels must burn rapidly and produce a high amount of energy. They should be combustible and capable of generating thrust effectively.

2. **Analyzing Option 1: Synthetic Rubber**:
- Synthetic rubber is known to burn rapidly and is a high-energy fuel. It is commonly used in some rocket propellants due to its favorable combustion characteristics. Therefore, synthetic rubber **is used as rocket fuel**.

3. **Analyzing Option 2: Liquid Hydrogen**:
- Liquid hydrogen is another fuel that burns rapidly and produces a high amount of energy when combusted with oxygen. It is widely used in many rocket systems, making it a suitable rocket fuel. Thus, liquid hydrogen **is used as rocket fuel**.

4. **Analyzing Option 3: Paraffin**:
- Paraffin wax is a solid fuel that has high regression rates, meaning it can burn efficiently and produce thrust. It is also utilized in some solid rocket propellants. Therefore, paraffin **is used as rocket fuel**.

5. **Analyzing Option 4: Liquid Nitrogen**:
- Liquid nitrogen does not burn; it is an inert gas at room temperature and does not produce energy through combustion. Additionally, it poses storage challenges due to its boiling point and the risk of pressure build-up. Hence, liquid nitrogen **is not used as rocket fuel**.

### Conclusion:
The option that is not used as a rocket fuel is **Liquid Nitrogen (Option 4)**.

---