Assertion : The outer electronic configuration of Cr and Cu are `3d^(5) 4s^(1) ` and `3d^(10) 4s^(1)` respectively .
Reason : Electrons are filled in orbitals in order of increasing energies given by (n+l) rule.

Options

A. If both assertion and reason are true and reason is the 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.

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that the outer electronic configurations of Chromium (Cr) and Copper (Cu) are \(3d^5 4s^1\) and \(3d^{10} 4s^1\) respectively.

- **Chromium (Cr)** has an atomic number of 24. Its electronic configuration can be derived as follows:
- The configuration starts with Argon (Ar), which has an atomic number of 18.
- The remaining 6 electrons are filled in the order of increasing energy levels.
- According to the expected filling order, it would be \(4s^2 3d^4\). However, to achieve a more stable half-filled \(3d\) subshell, one electron from \(4s\) is moved to \(3d\), resulting in \(3d^5 4s^1\).

- **Copper (Cu)** has an atomic number of 29. Its electronic configuration is:
- Starting with Argon (Ar) again, we have 11 electrons left to fill.
- The expected filling order would suggest \(4s^2 3d^9\). However, to achieve a fully filled \(3d\) subshell, one electron from \(4s\) is moved to \(3d\), resulting in \(3d^{10} 4s^1\).

Thus, the assertion is correct.

### Step 2: Analyze the Reason
The reason states that electrons are filled in orbitals in order of increasing energies given by the \(n+l\) rule.

- The \(n+l\) rule states that the order of filling orbitals is determined by the sum of the principal quantum number (n) and the azimuthal quantum number (l). The lower the value of \(n+l\), the lower the energy of the orbital.
- For \(4s\), \(n=4\) and \(l=0\), so \(n+l=4\).
- For \(3d\), \(n=3\) and \(l=2\), so \(n+l=5\).
- According to the \(n+l\) rule, \(4s\) should be filled before \(3d\), which is generally true.

However, the configurations of Chromium and Copper are exceptions to this rule due to stability considerations of half-filled and fully filled d orbitals.

### Conclusion
Both the assertion and the reason are true. However, the reason does not correctly explain the assertion since the configurations of Chromium and Copper are exceptions to the \(n+l\) rule.

### Final Answer
Both assertion and reason are true, but the reason is not a correct explanation of the assertion.