The least reactive element among the following is

Options

A. `""_(9)^(19)X`
B. `""_(10)^(20)Y`
C. `""_(11)^(23)X`
D. `""_(12)^(24)Y`

Explanation

To determine the least reactive element among the given options, we need to analyze the electronic configurations and the number of valence electrons for each element. The reactivity of an element is often related to its ability to achieve a stable electronic configuration, typically by completing its outer shell (octet).

### Step-by-Step Solution:

1. **Identify the Elements and Their Atomic Numbers**:
- Option 1: \(9X^{19}\) (Fluorine, atomic number 9)
- Option 2: \(10Y^{20}\) (Neon, atomic number 10)
- Option 3: \(11X^{23}\) (Sodium, atomic number 11)
- Option 4: \(12Y^{24}\) (Magnesium, atomic number 12)

2. **Determine the Electronic Configurations**:
- **Fluorine (9)**:
- Electronic configuration: \(2, 7\)
- Valence electrons: 7
- **Neon (10)**:
- Electronic configuration: \(2, 8\)
- Valence electrons: 8
- **Sodium (11)**:
- Electronic configuration: \(2, 8, 1\)
- Valence electrons: 1
- **Magnesium (12)**:
- Electronic configuration: \(2, 8, 2\)
- Valence electrons: 2

3. **Analyze Reactivity Based on Valence Electrons**:
- **Fluorine**: With 7 valence electrons, it needs to gain 1 electron to complete its octet, making it highly reactive.
- **Neon**: With 8 valence electrons, its octet is complete, making it non-reactive or the least reactive.
- **Sodium**: With 1 valence electron, it will lose that electron to achieve a stable configuration, making it reactive.
- **Magnesium**: With 2 valence electrons, it will lose 2 electrons to achieve a stable configuration, making it reactive.

4. **Conclusion**:
- Among the options, **Neon (10Y^{20})** is the least reactive element because it has a complete octet (8 valence electrons) and does not need to gain or lose electrons to achieve stability.

### Final Answer:
The least reactive element among the following is **Option 2: 10Y^{20} (Neon)**.