The particles which cannot be accelerated by a cyclotron or a Van de Graff generator are :

Options

A. Alpha particles
B. Beta particles
C. Neutrons
D. Protons

Explanation

To solve the question regarding which particles cannot be accelerated by a cyclotron or a Van de Graaff generator, we need to analyze the properties of the particles listed in the options.

### Step-by-Step Solution:

1. **Understanding the Devices**:
- A **cyclotron** and a **Van de Graaff generator** are both devices used to accelerate charged particles. They rely on electric and magnetic fields to impart energy to particles.

2. **Identifying the Particles**:
- The options given are:
1. Alpha particles
2. Beta particles
3. Neutrons
4. Protons

3. **Analyzing Each Particle**:
- **Alpha Particles**: These are positively charged particles (consisting of 2 protons and 2 neutrons). They can be accelerated by both devices.
- **Beta Particles**: These are charged particles (electrons or positrons). They can also be accelerated by both devices.
- **Protons**: These are positively charged particles. They can be accelerated by both devices as well.
- **Neutrons**: These are neutral particles and do not have any charge. Since cyclotrons and Van de Graaff generators require a charge to accelerate particles, neutrons cannot be accelerated by either device.

4. **Conclusion**:
- Since neutrons are the only neutral particles among the options, they cannot be accelerated by a cyclotron or a Van de Graaff generator. Therefore, the correct answer is **Option 3: Neutrons**.

### Final Answer:
The particles which cannot be accelerated by a cyclotron or a Van de Graaff generator are **Neutrons**.