A: `K_(4)[Fe(CN)_(6)]`is less stable than `K_(3)[Fe(CN)_(6)]`
R: In `K_(4)Fe(CN)_(6)]` the EAN of Fe is 36

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. ) If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to analyze the assertion and reason given:

**Assertion (A):** `K4[Fe(CN)6]` is less stable than `K3[Fe(CN)6]`.
**Reason (R):** In `K4[Fe(CN)6]`, the Effective Atomic Number (EAN) of Fe is 36.

### Step-by-Step Solution:

1. **Understanding the Compounds:**
- `K4[Fe(CN)6]` and `K3[Fe(CN)6]` are coordination compounds where Fe is the central metal ion and CN is the ligand.

2. **Determining the Oxidation State of Fe:**
- For `K4[Fe(CN)6]`:
- The formula can be dissociated as `4K^+` and `[Fe(CN)6]^{4-}`.
- Let the oxidation state of Fe be \( x \).
- The oxidation state of CN is -1, and there are 6 CN ligands, thus:
\[ x + 6(-1) = -4 \implies x - 6 = -4 \implies x = +2 \]
- For `K3[Fe(CN)6]`:
- The formula can be dissociated as `3K^+` and `[Fe(CN)6]^{3-}`.
- Again, let the oxidation state of Fe be \( x \):
\[ x + 6(-1) = -3 \implies x - 6 = -3 \implies x = +3 \]

3. **Comparing the Oxidation States:**
- In `K4[Fe(CN)6]`, Fe has an oxidation state of +2.
- In `K3[Fe(CN)6]`, Fe has an oxidation state of +3.
- A higher oxidation state generally leads to a higher charge density, which increases the stability of the complex.

4. **Conclusion on Stability:**
- Since `K3[Fe(CN)6]` has a higher oxidation state (+3) compared to `K4[Fe(CN)6]` (+2), it is more stable.
- Therefore, the assertion that `K4[Fe(CN)6]` is less stable than `K3[Fe(CN)6]` is correct.

5. **Calculating the Effective Atomic Number (EAN):**
- The formula for EAN is given by:
\[ \text{EAN} = Z - \text{oxidation state} + 8 \times \text{coordination number} \]
- For Fe (atomic number \( Z = 26 \)) in `K4[Fe(CN)6]`:
- Oxidation state = +2
- Coordination number = 6
\[ \text{EAN} = 26 - 2 + 8 \times 6 = 26 - 2 + 48 = 72 \]
- The statement in the reason that the EAN of Fe is 36 is incorrect.

6. **Final Evaluation:**
- The assertion is true, but the reason provided is not a correct explanation for the assertion because the EAN calculated does not match the claim in the reason.

### Final Answer:
- The assertion is correct, but the reason is incorrect. Therefore, the correct option is **B**.