Answer: Option (a) is the correct answer.
Step-by-step explanation:
Amount of energy needed to remove an electron from a neutral or gaseous atom is known as ionization energy.
The energy required to remove one valence electron from a neutral or gaseous atom is called first ionization energy whereas when two valence electrons are removed then it is called second ionization energy.
Thus, we can conclude that the energy required to remove two valency electrons of a metallic atom is called second ionization energy.