The rule that explains the reason for $Cr$ to have $\left[Ar\right]$$3d^5$$4s^1$ instead of $\left[Ar\right]$$3d^4$$4s^2$:

The correct option is C Stability of half filled and completely filled orbitals
It is due to the fact that completely filled or half-filled orbital is always lower in energy and therefore, are more stable.
According to the Aufbau rule, electronic configuration of $Cr$ could be $\left[Ar\right]3d^4$$4s^2$. But to achieve half filled configuration, one electron moves from filled $4s$ subshell to $3d$ subshell, and gives rise to more stable $\left[Ar\right]3d^{5}4s^1$ configuration.