The set representing the correct order of the first ionisation potential is

Options

A. `K gt Na gt Li`
B. `Be gt Mg gt Ca`
C. `B gt C gt N`
D. `Ge gt Si gt C`

Explanation

To determine the correct order of the first ionization potential among the given sets, we need to understand the trends in ionization energy based on the periodic table. Here’s a step-by-step solution:

### Step 1: Understand Ionization Energy Trends
Ionization energy is the energy required to remove an electron from an atom. The trends are as follows:
- **Down a Group**: Ionization energy decreases because the atomic size increases, making it easier to remove an outer electron.
- **Across a Period**: Ionization energy increases from left to right due to increasing nuclear charge and decreasing atomic size.

### Step 2: Analyze Each Option
We will analyze each option provided in the question to determine if they follow the trends mentioned above.

#### Option A: Lithium (Li), Sodium (Na), Potassium (K)
- These elements are in the same group (Group 1).
- As we move down the group from Li to Na to K, the atomic size increases, and thus the ionization energy decreases.
- Correct order should be: **Li > Na > K**.
- **Conclusion**: This option is incorrect.

#### Option B: Beryllium (Be), Magnesium (Mg), Calcium (Ca)
- These elements are in the same group (Group 2).
- Following the same trend, as we move down from Be to Mg to Ca, the atomic size increases, and ionization energy decreases.
- Correct order should be: **Be > Mg > Ca**.
- **Conclusion**: This option is correct.

#### Option C: Boron (B), Carbon (C), Nitrogen (N)
- These elements are in the same period (Period 2).
- As we move from B to C to N, the atomic size decreases, and ionization energy increases.
- Correct order should be: **N > C > B**.
- **Conclusion**: This option is incorrect.

#### Option D: Carbon (C), Silicon (Si), Germanium (Ge)
- These elements are in the same group (Group 14).
- As we move down from C to Si to Ge, the atomic size increases, and ionization energy decreases.
- Correct order should be: **C > Si > Ge**.
- **Conclusion**: This option is incorrect.

### Step 3: Final Conclusion
After analyzing all options, the only correct order of first ionization potential is found in **Option B: Be > Mg > Ca**.

### Final Answer:
The correct order of the first ionization potential is **Option B**.