Assertion. Only principal quantum number determines the energy of an electron in an orbital of Na atom.
Reason. For one electron system, the expression of energy is quite different from that obtained in Bohr's theory.

Options

A. If both assertion and reason are true, and reason is the true explanation of the assertion
B. If both assertion and reason are true, but reason is not the true explanation of the assertion
C. If assertion is true, but reason is false
D. If both assertion and reason are false

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts of quantum numbers, energy levels, and the specifics of the sodium (Na) atom.

### Step-by-Step Solution:

1. **Understanding Quantum Numbers**:
- The principal quantum number (n) indicates the main energy level of an electron in an atom. It can take positive integer values (1, 2, 3, ...).
- The azimuthal quantum number (l) indicates the shape of the orbital and can take values from 0 to (n-1).

2. **Energy Determination in Multi-Electron Atoms**:
- In multi-electron atoms, such as sodium (Na), the energy of an electron is influenced not only by the principal quantum number (n) but also by the azimuthal quantum number (l).
- The energy levels are affected by electron-electron interactions and shielding effects, which means that electrons in different orbitals (with different l values) at the same n level can have different energies.

3. **Assertion Evaluation**:
- The assertion states that "only the principal quantum number determines the energy of an electron in an orbital of Na atom." This is **false** because, in multi-electron systems, both n and l influence the energy of electrons.

4. **Reason Evaluation**:
- The reason states that "for one-electron systems, the expression of energy is quite different from that obtained in Bohr's theory." This is **true**. In a one-electron system (like the hydrogen atom), the energy levels can be accurately described by Bohr's model, where energy depends solely on n. However, for multi-electron atoms, the energy levels are more complex due to additional interactions.

5. **Conclusion**:
- Since the assertion is false and the reason is true, the overall statement is incorrect. The principal quantum number alone does not determine the energy of electrons in sodium; both n and l are significant.

### Final Answer:
- The assertion is false, and the reason is true. Therefore, the statement is incorrect.