Explanation:

First ionization energy is the energy required to remove one electron from an atom. The factors affecting the first ionization energy of elements are:

- Atomic radius: The larger the atomic radius, the lower the ionization energy.
- Nuclear charge: The higher the nuclear charge, the higher the ionization energy.
- Electron shielding: The more the number of electrons in inner shells, the lower the ionization energy.

Among the given options, the correct answer is option 'D', Tin. This is because of the following reasons:

- Atomic radius: Tin has a larger atomic radius than carbon and silicon, and almost the same as lead. Therefore, it is not affected by atomic radius.
- Nuclear charge: Tin has a nuclear charge of 50, which is higher than carbon and silicon, but lower than lead. Therefore, it is affected by nuclear charge, but not as much as lead.
- Electron shielding: Tin has 5 electrons in its inner shells, which shield the outermost electron from the nuclear charge. This makes it easier to remove the outermost electron, and hence, the first ionization energy of tin is lower than lead.

Therefore, the first ionization energy is lowest for Tin among the given options.