Final answer:
Potassium is the metal that loses electrons most readily out of the options provided, as it's an alkali metal with only one valence electron that it can easily lose to form a K+ cation.
Step-by-step explanation:
Among the metals listed - calcium, magnesium, potassium, and sodium - the metal that loses electrons most readily is potassium. This is because potassium is an alkali metal from group 1 of the periodic table, where elements have only one electron in their valence shell. This single electron is easier to remove compared to the two electrons in the valence shell of alkaline earth metals like calcium and magnesium. As such, potassium can form a cation, specifically a K+ cation, by losing one electron, which results in a configuration isoelectronic with the noble gas argon.
By contrast, magnesium, for instance, needs to lose two electrons to form a Mg2+ ion, which is isoelectronic with neon. Calcium, another alkaline earth metal, similarly loses two electrons to form a Ca2+ ion. While sodium is also an alkali metal and loses one electron to form Na+, it is less reactive than potassium due to its smaller size and higher ionization energy.