The correct answer is a. shielding

Explanation:

The addition of electrons causes the atomic size to increase due to the addition of new electronic shells. Due to the increase in atomic size, electrons become more away from the nucleus as many other electrons are present in the way from the nucleus to the outer electrons.

The hold of the nucleus becomes weaker. The nuclear charge is also increased at the same time, but the inner electrons shield the outer electrons. So effective nuclear charge is weaker than the actual nuclear charge.

Because of this shielding, it is easy to remove the electrons or we can say ionization energy decreases.