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 first ionization potentials from the given options, we need to understand the trends in ionization energy across periods and down groups in the periodic table.

### Step-by-Step Solution:

1. **Understand Ionization Energy**:
- Ionization energy (or ionization potential) is the energy required to remove an electron from an atom in its gaseous state.
- It generally increases across a period (from left to right) due to increasing nuclear charge and decreasing atomic radius.
- It generally decreases down a group due to increasing atomic size and shielding effect, which makes it easier to remove an outer electron.

2. **Analyze the Given Sets**:
- We need to evaluate the provided sets of elements to see if they follow the trends mentioned above.

3. **Evaluate Each Set**:
- **Set 1**: (e.g., Lithium, Beryllium, etc.) - If this set goes down a group, we expect the ionization energy to decrease. If it shows an increase, this is incorrect.
- **Set 2**: (e.g., Beryllium, Magnesium, Calcium) - This set represents elements going down Group 2. According to the trend, ionization energy should decrease. This set is correct.
- **Set 3**: (e.g., Sodium, Magnesium, etc.) - If this set goes across a period and shows a decrease in ionization energy, it is incorrect.
- **Set 4**: (e.g., Nitrogen, Oxygen, etc.) - If this set goes up a group and shows a decrease, it is also incorrect.

4. **Identify the Correct Order**:
- From the analysis, the only set that correctly represents the trend in ionization energy is the second set (Beryllium, Magnesium, Calcium), where ionization energy decreases as we move down the group.

5. **Conclusion**:
- The correct order of first ionization potential is represented by the second set of elements.