Final answer:
Magnesium (Mg) has the highest ionization energy among the given options because it lies farthest to the right and not the furthest down in the periodic table.
Step-by-step explanation:
The element that has the highest ionization energy among the options given is magnesium (Mg). Ionization energy generally increases across a period from left to right due to increasing effective nuclear charge, and decreases down a group as the outer electrons are farther from the nucleus.
Among sodium (Na), magnesium (Mg), potassium (K), and calcium (Ca), Mg is farthest to the right and not the farthest down in the periodic table, indicating it has the highest ionization energy. This can also be rationalized by considering that Mg has a smaller atomic radius and a greater effective nuclear charge than the other options, making it more difficult to remove an electron.