As we go down in a group, its ionization energy decreases and
as we move from left to right along a period its ionization energy increases.
It may have some exceptions which u can get yourself clarrified from NCERT.

**Introduction:**
Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous state. It is influenced by various factors such as atomic radius, nuclear charge, and electron shielding. The atom with the highest first ionization energy will have the most tightly held electrons.

**Explanation:**
To determine which atom has the highest first ionization energy among the given options, let's analyze the factors that affect ionization energy for each atom.

1. **Atomic Radius:**
The larger the atomic radius, the weaker the attraction between the nucleus and the outermost electron. This results in a lower ionization energy. Conversely, a smaller atomic radius leads to a higher ionization energy.

2. **Nuclear Charge:**
The greater the nuclear charge (number of protons in the nucleus), the stronger the attraction between the nucleus and the electrons, resulting in a higher ionization energy.

3. **Electron Shielding:**
Electron shielding refers to the repulsion between electrons in different energy levels. The more electron shielding there is, the weaker the attraction between the nucleus and the outermost electron, leading to a lower ionization energy.

Now, let's examine the given options and determine which atom has the highest first ionization energy.

- **Option A: Lithium (Li)**
Lithium has a relatively low atomic number (3) and is located in the second period of the periodic table. It has a larger atomic radius compared to the other options, which indicates a weaker attraction between the nucleus and the outermost electron. Therefore, lithium has a lower first ionization energy compared to the other options.

- **Option B: Beryllium (Be)**
Beryllium has a slightly higher atomic number (4) compared to lithium. It is also located in the second period of the periodic table. Beryllium has a smaller atomic radius than lithium, indicating a stronger attraction between the nucleus and the outermost electron. Consequently, beryllium has a higher first ionization energy than lithium.

- **Option C: Boron (B)**
Boron has an atomic number of 5 and is located in the second period of the periodic table. It has a smaller atomic radius than beryllium due to the increased nuclear charge. This results in a stronger attraction between the nucleus and the outermost electron, leading to a higher first ionization energy than beryllium.

- **Option D: Carbon (C)**
Carbon has an atomic number of 6 and is also located in the second period of the periodic table. It has a smaller atomic radius than boron due to the increased nuclear charge. Carbon has the highest nuclear charge among the given options, resulting in the strongest attraction between the nucleus and the outermost electron. Therefore, carbon has the highest first ionization energy among the given options.

**Conclusion:**
Among the given options, carbon (C) has the highest first ionization energy. This is because carbon has a smaller atomic radius, a higher nuclear charge, and stronger attraction between the nucleus and the outermost electron compared to the other options.