The correct ground state electronic configuration of a chromium (Cr) atom is Option A: [Ar] 3d⁵ 4s¹.

To understand why, we need to recall that chromium is an exception to the general rule of electron configurations. Normally, we would follow the Aufbau principle to sequentially fill the 4s orbital before the 3d orbitals. This would suggest an expected configuration similar to Option B, [Ar] 3d⁴ 4s², for a chromium atom. However, chromium is an exception due to the extra stability provided by a half-filled d subshell.

Electron configurations for elements are determined by the principles of lower energy and higher stability. Half-filled and fully-filled subshells (like 3d⁵ and 3d¹⁰) are particularly stable configurations. For chromium, the configuration [Ar] 3d⁵ 4s¹ offers a lower energy state compared to a completely filled 4s orbital and a less-than-half-filled 3d orbital. By having five electrons in the 3d subshell and only one in the 4s, both stability and a lower energy configuration are achieved.

Therefore, the correct answer is:

Option A: [Ar] 3d⁵ 4s¹.