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.

Options

A. If both Assertion and Reason are true and Reason is correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason are false.

Answer

To solve the question, we need to analyze the assertion and the reason provided.

### Step-by-Step Solution

1. **Understanding the Forces**:
- The assertion states that the forces acting between protons (f_pp), protons and neutrons (f_pn), and neutrons (f_nn) are such that:
\[ f_{pp} < f_{pn} = f_{nn} \]
- We need to understand the nature of these forces. The force between two protons is influenced by their positive charge, which leads to electrostatic repulsion.

2. **Force Between Protons (f_pp)**:
- Since protons are positively charged, the electrostatic force between two protons is repulsive. Thus, the net force acting between two protons can be expressed as:
\[ f_{pp} = N - E \]
where \(N\) is the attractive nuclear force and \(E\) is the electrostatic repulsive force. The repulsion reduces the overall attractive force, making \(f_{pp}\) less than the other forces.

3. **Force Between Proton and Neutron (f_pn)**:
- The proton is positively charged while the neutron is neutral. Therefore, there is no electrostatic force acting between them. The force between a proton and a neutron is purely due to the nuclear force:
\[ f_{pn} = N \]

4. **Force Between Neutrons (f_nn)**:
- Neutrons are neutral, so there is no electrostatic force between two neutrons. The force acting between them is also purely due to the nuclear force:
\[ f_{nn} = N \]

5. **Comparing the Forces**:
- From the above analysis, we can summarize:
- \(f_{pp} = N - E\) (less than \(N\))
- \(f_{pn} = N\)
- \(f_{nn} = N\)
- Thus, we can conclude that:
\[ f_{pp} < f_{pn} = f_{nn} \]
- This confirms the assertion is true.

6. **Analyzing the Reason**:
- The reason states that the electrostatic force of repulsion between two protons reduces the net nuclear force between them. This is consistent with our findings since the repulsive force indeed subtracts from the attractive nuclear force, leading to a lower net force between protons.

7. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer
Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.