Assertion: Forces acting between proton-proton `(f_(pp))`, proton -neutron `(f_(pn))` and neutron -neutron `(f_(n n))` are such that `f_(pp) lt f_(pn)=f_(n n)`.
Reason: Electrostatic force of repulsion between two proton reduces net nuclear forces between them.

Explanation

To solve the question, we need to analyze the assertion and reason provided regarding the forces acting between protons and neutrons in an atomic nucleus.

### Step-by-Step Solution:

1. **Understanding the Forces**:
- The forces acting between protons (f_pp), protons and neutrons (f_pn), and neutrons (f_nn) are influenced by both nuclear forces and electrostatic forces.
- Protons are positively charged and experience electrostatic repulsion when they are close to each other.

2. **Analyzing the Assertion**:
- The assertion states that \( f_{pp} < f_{pn} = f_{nn} \).
- For proton-proton interaction (f_pp):
- There is an electrostatic repulsive force due to their positive charges, which reduces the effective nuclear force between them.
- Thus, \( f_{pp} \) is less than the forces between proton-neutron and neutron-neutron.

3. **Analyzing the Proton-Neutron Interaction**:
- For proton-neutron interaction (f_pn):
- The proton is positively charged, and the neutron is neutral, so there is no electrostatic force acting between them.
- The force here is purely due to the attractive nuclear force.

4. **Analyzing the Neutron-Neutron Interaction**:
- For neutron-neutron interaction (f_nn):
- Both particles are neutral, so again, there is no electrostatic force.
- The force is purely due to the attractive nuclear force.

5. **Comparing the Forces**:
- Since \( f_{pp} \) is reduced by the electrostatic repulsion, it is less than both \( f_{pn} \) and \( f_{nn} \), which are equal because both involve only nuclear forces.
- Therefore, the assertion \( f_{pp} < f_{pn} = f_{nn} \) is correct.

6. **Understanding the Reason**:
- The reason states that the electrostatic force of repulsion between two protons reduces the net nuclear forces between them.
- This is accurate because the repulsion indeed diminishes the effective attractive nuclear force, leading to the conclusion drawn in the assertion.

7. **Conclusion**:
- Both the assertion and the reason are correct, and the reason accurately explains the assertion.

### Final Answer:
The correct answer is option A: Assertion is correct, Reason is correct, and Reason is able to correctly explain Assertion.