Which of the following has the electron configuration of [Ar] 4s^1?

The element with the electron configuration of [Ar] 4s^1 is potassium. To understand this, we should look at how electron configurations are constructed based on the arrangement of electrons in an atom.

In this case, the notation [Ar] represents the electron configuration of argon, which contains 18 electrons. After argon, the next electron shell begins to fill with the 4s subshell. Potassium, which has an atomic number of 19, has one additional electron beyond argon, leading to the electron configuration of [Ar] 4s^1. This indicates that there is one electron in the 4s subshell.

The choices provided include elements with different atomic numbers and therefore different electron configurations. Calcium, magnesium, and sodium have electron configurations that differ in how their electrons are arranged, specifically in their outermost shells and subshells. Calcium and magnesium have filled their 4s subshell with two and three electrons, respectively, while sodium only has one electron in the 3s subshell. These configurations reflect their positions and properties in the periodic table, but only potassium fits the specific configuration of [Ar] 4s^1, which correctly identifies it as the element being asked about