Answer: B. Mg
Step-by-step explanation:
Ionization energy is the energy required for the removal of valence electron from an isolated gaseous atom.
Ionization energy increases as we move across the period as the atomic size decreases as the electrons are being added to the same energy level.
Ionization energy decreases as we move down the group as the atomic size increases due to the addition of shells.
As magnesium (Mg) lies to the right of Sodium (Na) and Poatssium (K) and Calcium (Ca) lies below Magnesium (Mg), Mg has the highest ionization energy.