Final answer:
Potassium (K) has the lowest first ionization energy among the elements Na, K, Mg, and Ca due to its position lower in the group and further to the left in the periodic table.
Step-by-step explanation:
The element with the lowest first ionization energy among Na, K, Mg, and Ca is potassium (K). The term first ionization energy refers to the energy required to remove the most loosely held electron from an atom in its gaseous state. According to periodic trends, ionization energy decreases from top to bottom within a group, and increases from left to right across a period. Potassium is located lower in the group than sodium, magnesium, and calcium, indicating that it will have a lower first ionization energy. Moreover, it is located further to the left than magnesium and calcium. Therefore, based on its position in the periodic table, potassium (K) has the lowest ionization energy of the four elements listed.