The massive particle among the following is

Options

A. `alpha`-particle
B. Deuteron
C. Proton
D. `beta`-particle

Explanation

To determine which particle is the most massive among the given options, we will analyze the mass of each particle mentioned: the alpha particle, deuteron, and beta particle.

### Step-by-Step Solution:

1. **Identify the Particles**:
- **Alpha Particle**: An alpha particle is a helium nucleus, which consists of 2 protons and 2 neutrons. Its mass is approximately 4 atomic mass units (u).
- **Deuteron**: A deuteron is a hydrogen isotope that contains 1 proton and 1 neutron. Its mass is approximately 2 atomic mass units (u).
- **Beta Particle**: A beta particle is an electron or positron. Its mass is negligible compared to protons and neutrons, approximately 0.0005 atomic mass units (u).

2. **Compare the Masses**:
- Alpha Particle: 4 u
- Deuteron: 2 u
- Beta Particle: ~0.0005 u

3. **Determine the Most Massive Particle**:
- Among the three particles, the alpha particle has the highest mass at 4 u, followed by the deuteron at 2 u, and the beta particle is significantly lighter.

4. **Conclusion**:
- Therefore, the most massive particle among the given options is the **alpha particle**.

### Final Answer:
The most massive particle is the **alpha particle**.

---