In nuclear reactions, we have the conservation of

Options

A. Mass only
B. Energy only
C. Momentum only
D. Mass, energy and momentum

Explanation

To answer the question about what is conserved in nuclear reactions, we can break it down step by step:

### Step-by-Step Solution:

1. **Understanding Nuclear Reactions**:
Nuclear reactions involve changes in the nucleus of atoms, where one type of nucleus transforms into another. This can involve the fusion of nuclei, fission of heavy nuclei, or radioactive decay.

2. **Conservation of Mass**:
In nuclear reactions, the total mass number (the sum of protons and neutrons) before the reaction must equal the total mass number after the reaction. For example, in the reaction involving Uranium-235 absorbing a neutron, we can calculate the mass numbers on both sides of the equation to confirm this conservation.

3. **Conservation of Charge**:
The total charge (number of protons) must also be conserved in nuclear reactions. The number of protons in the reactants must equal the number of protons in the products. In our example, we see that the total number of protons remains the same before and after the reaction.

4. **Conservation of Energy**:
Energy is conserved in nuclear reactions as well. The energy released or absorbed during the reaction must be accounted for, and the total energy before the reaction equals the total energy after the reaction.

5. **Conservation of Momentum**:
Momentum is also conserved in nuclear reactions. The total momentum of the system before the reaction must equal the total momentum after the reaction.

6. **Conclusion**:
Therefore, in nuclear reactions, we have the conservation of mass, charge, energy, and momentum.

### Final Answer:
In nuclear reactions, we have the conservation of mass, charge, energy, and momentum.