The law of conservation of mass holds good for all of the following except.

Options

A. All chemical reactions
B. Nuclear reactions
C. Endothermic reactions
D. Exothermic reaction

Answer

To solve the question regarding the law of conservation of mass, we will analyze each option provided and determine which one does not adhere to this law.

### Step-by-Step Solution:

1. **Understanding the Law of Conservation of Mass**:
- The law states that mass cannot be created or destroyed in a chemical reaction. Therefore, the total mass of reactants must equal the total mass of products.

2. **Analyzing Option A: All Chemical Reactions**:
- In chemical reactions, the law of conservation of mass holds true. The mass of the reactants is equal to the mass of the products. Thus, this option does not violate the law.

**Conclusion**: Option A is not the answer.

3. **Analyzing Option B: Nuclear Reactions**:
- Nuclear reactions involve changes in the nucleus of atoms, where mass can be converted into energy according to Einstein's equation \(E=mc^2\). This means that in nuclear reactions, some mass is lost and converted into energy, violating the law of conservation of mass.

**Conclusion**: Option B is a strong candidate for the answer.

4. **Analyzing Option C: Endothermic Reactions**:
- Endothermic reactions absorb energy from the surroundings but do not change the total mass of the reactants and products. They still adhere to the law of conservation of mass.

**Conclusion**: Option C is not the answer.

5. **Analyzing Option D: Exothermic Reactions**:
- Exothermic reactions release energy, but like endothermic reactions, they also do not violate the law of conservation of mass. The mass of the reactants equals the mass of the products.

**Conclusion**: Option D is not the answer.

6. **Final Conclusion**:
- The only option that does not adhere to the law of conservation of mass is **Option B: Nuclear Reactions**.

### Answer:
**The law of conservation of mass holds good for all of the following except: Option B - Nuclear Reactions.**