Concept:High ionization energy means a large amount of energy is needed to remove an electron because the nucleus holds the electrons very strongly.
Explanation:A high effective nuclear charge means the positively charged nucleus exerts a stronger pull on the surrounding electrons.
This stronger attraction makes it much harder to remove an electron from the atom.
Therefore, elements with high ionization energies do not lose electrons easily; they lose them with difficulty.
Such atoms also tend to have smaller atomic radii, not larger ones, because the electrons are pulled closer to the nucleus.
The key reason for high ionization energy is therefore a high effective nuclear charge, not low atomic number or large atomic size.
Among the given options, high effective nuclear charge directly explains why the ionization energy is high.
Answer:C. have high effective nuclear charges