`K^(o+)` ion is isolectronic with

Options

A. `Na^(o+)`
B. `Ne`
C. `Ar`
D. `Cs^(o+)`

Explanation

To determine which ion or element is isoelectronic with the \( K^{+} \) ion, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Atomic Number of Potassium (K)**:
- Potassium (K) has an atomic number of 19.

2. **Determine the Electron Configuration of \( K^{+} \)**:
- The \( K^{+} \) ion is formed when potassium loses one electron.
- Therefore, the number of electrons in \( K^{+} \) is \( 19 - 1 = 18 \).

3. **Find Elements or Ions with 18 Electrons**:
- We need to find another element or ion that also has 18 electrons to be isoelectronic with \( K^{+} \).

4. **Evaluate the Given Options**:
- **Option 1: Sodium (Na)**:
- Sodium has an atomic number of 11. When it loses one electron to form \( Na^{+} \), it has \( 11 - 1 = 10 \) electrons. Not isoelectronic.
- **Option 2: Neon (Ne)**:
- Neon has an atomic number of 10. It has 10 electrons and does not change when it forms a neutral atom. Not isoelectronic.
- **Option 3: Argon (Ar)**:
- Argon has an atomic number of 18. It has 18 electrons and is neutral. Therefore, it is isoelectronic with \( K^{+} \).
- **Option 4: Cesium (Cs)**:
- Cesium has an atomic number of 55. When it loses one electron to form \( Cs^{+} \), it has \( 55 - 1 = 54 \) electrons. Not isoelectronic.

5. **Conclusion**:
- The only element that is isoelectronic with \( K^{+} \) is Argon (Ar), which has 18 electrons.

### Final Answer:
The \( K^{+} \) ion is isoelectronic with Argon (Ar).