(A) : The electronic configuration of Cr in the ground state is $\left[\mathrm{Ar}\right]4{\mathrm{s}}^{1}3{\mathrm{d}}^5$ Instead of $\left[\mathrm{Ar}\right]4{\mathrm{s}}^{2}3{\mathrm{d}}^4$
(R) : The average exchange energy associated with 4s¹3d⁵ configuration is less when compared to that of 4s²3d⁴

The electronic configuration of Cr is $\left[\mathrm{Ar}\right]4{\mathrm{s}}^{1}3{\mathrm{d}}^5(\mathrm{half} \mathrm{filled} \mathrm{configuration})$

This is due to the stability of half filled configuration.

The stability is due to symmetry and exchange energy.

As no. of unpaired electrons(or parallel spin e^-s) increases, exchange energy increases and stability increases.

In 4s¹3d⁵ ,  no. of unpaired electrons=6

In 4s²3d⁴ ,  no. of unpaired electrons=4