Final answer:
The reaction with the highest first ionization energy is Mg + energy -> Mg+ + e-, because magnesium has a higher nuclear charge than sodium, potassium, and calcium, and thus its valence electrons are more strongly attracted to the nucleus.
Step-by-step explanation:
To determine which reaction has the greatest first ionization energy, we must consider the atomic structure and the energy required to remove an electron from a neutral atom.
Ionization energy tends to increase across a period and decrease down a group in the periodic table.
For the reactions given: Na, K, Mg, and Ca, Mg (C. Mg + energy -> Mg+ + e-) would have the highest first ionization energy, as Mg is located to the right of Na and K in the same period, indicating a stronger nuclear charge that more tightly holds its valence electrons.
Mg also has a higher nuclear charge than Ca but the same number of electron shells, which means Mg's valence electrons are more strongly attracted to the nucleus than Ca's due to the higher effective nuclear charge.
Therefore, Mg will require more energy to remove an electron compared to Na, K, or Ca.