Assertion (A): The electronic configuration of Cr in the ground state is [Ar]$4s^{1}3d^5$ instead of [Ar]$4s^{2}3d^4$
Reason (R): The Average exchange energy associated with $4s^{1}3d^5$ configuration is more when compared to that of [Ar]$4s^{2}3d^4$

However, we also know, half filled is more stable than partially filled orbitals. Also, 3d⁵ is more stable and less energy than 4s² configuration. In case of Chromium, after the 4s² 3d⁴ configuration is attained, an electron from the 4s orbital jumps to 3d subshell because 3d⁵ is a much more stable configuration than 3d⁴. That's why final configuration for Chromium is 4s¹ 3d⁵ and not 3d⁴ 4s².