(a). IF both (A) and (R) are correct and (R) is the correct explanation of (A).
(b). If both (A) and (R) are correct but (R) is not the correct explanation of (A).
(c). If (A) is correct, but (R) is incorrect.
(d). If (A) is incorrect, but (R) is correct.
(e) if both (A) and (R) are incorrect.
Assertion (A): if x " mL of " 0.1 M `KMnO_(4)` solution requires 100 " mL of " 0.1 M CuS solution for complete neutralisation then x " mL of " same `KMnO_(4)` solution would require `75` " mL of " 0.1 M `Cu_(2)S` solution.
Reason (R): n factor for Cus and `Cu_(2)S` respectively are 6 and 8.

Explanation

To solve the assertion and reason question, we need to analyze both the assertion (A) and the reason (R) step by step.

### Step 1: Analyze the Assertion (A)

**Assertion (A):** If x mL of 0.1 M KMnO4 solution requires 100 mL of 0.1 M CuS solution for complete neutralization, then x mL of the same KMnO4 solution would require 75 mL of 0.1 M Cu2S solution.

1. **Identify the Reaction:**
- KMnO4 reacts with CuS.
- The relevant half-reaction for KMnO4 is:
\[ \text{MnO}_4^- + 8 \text{H}^+ + 5 \text{e}^- \rightarrow \text{Mn}^{2+} + 4 \text{H}_2\text{O} \]
- The n-factor for KMnO4 in this reaction is 5.

2. **Calculate Equivalents for CuS:**
- The reaction of CuS can be represented as:
\[ \text{CuS} \rightarrow \text{Cu}^{2+} + \text{S}^{6+} \]
- The n-factor for CuS is determined by the change in oxidation state of sulfur, which is +6 for sulfate and -2 for sulfide, resulting in a change of 8 electrons.

3. **Set Up the Equation:**
- Using the equivalence concept:
\[ \text{Equivalents of KMnO}_4 = \text{Equivalents of CuS} \]
- For KMnO4:
\[ \text{Equivalents} = n \times \text{Molarity} \times \text{Volume} = 5 \times 0.1 \times x \]
- For CuS:
\[ \text{Equivalents} = n \times \text{Molarity} \times \text{Volume} = 8 \times 0.1 \times 100 \]

4. **Equating the Two:**
\[ 5 \times 0.1 \times x = 8 \times 0.1 \times 100 \]
Simplifying gives:
\[ 5x = 800 \implies x = 160 \text{ mL} \]

### Step 2: Analyze the Reason (R)

**Reason (R):** The n-factor for CuS and Cu2S are 6 and 8, respectively.

1. **Determine n-factors:**
- For CuS, the n-factor is indeed 8 (as derived above).
- For Cu2S, the oxidation changes for both copper and sulfur need to be considered:
- Copper changes from +1 to +2 (1 electron per copper, 2 copper atoms = 2 electrons).
- Sulfur changes from -2 to +6 (8 electrons).
- Therefore, the total n-factor for Cu2S is 10, not 8.

### Step 3: Conclusion

- **Assertion (A) is Incorrect:** The assertion states that 75 mL of Cu2S is required, but we calculated that 80 mL is needed.
- **Reason (R) is Incorrect:** The n-factors provided (6 for CuS and 8 for Cu2S) are incorrect; the correct n-factors are 8 for CuS and 10 for Cu2S.

### Final Answer

Both (A) and (R) are incorrect.