The cation energy of an electron is largely determined by its principal quantum number.
The principal quantum number `n` is a measure of the most probable distance of finding atomic the electron around the nucleus.

Options

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

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "the cation energy of an electron is largely determined by its principal quantum number."
- This is true because the energy of an electron in an atom can be calculated using the formula:
\[ E = -\frac{13.6 \, Z^2}{n^2} \text{ eV} \]
where \(E\) is the energy, \(Z\) is the atomic number, and \(n\) is the principal quantum number. As \(n\) increases, the energy becomes less negative, indicating that the electron is less tightly bound to the nucleus.

2. **Understanding the Reason**:
- The reason states that "the principal quantum number \(n\) is a measure of the most probable distance of finding an atomic electron around the nucleus."
- This is also true. The principal quantum number \(n\) does relate to the size of the atomic orbital. The average distance of the electron from the nucleus can be expressed as:
\[ r = 0.529 \times \frac{n^2}{Z} \]
where \(r\) is the radius of the atomic orbital. As \(n\) increases, the average distance of the electron from the nucleus increases.

3. **Evaluating the Relationship**:
- Both the assertion and the reason are true statements. Furthermore, the reason provides a correct explanation for the assertion. The principal quantum number \(n\) not only determines the energy of the electron but also indicates the average distance of the electron from the nucleus.

4. **Conclusion**:
- Since both the assertion and reason are true and the reason correctly explains the assertion, the correct answer is:
**Option A: Assertion and reason are true. Reason is correct explanation of assertion.**