The expected electronic configuration of $\ce{Cr}$ ($Z = 24$) is $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^4\,4s^2$, but the actual configuration is:

• a$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5\,4s^1$
• b$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^3\,4s^3$
• c$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6$
• d$1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^2\,3d^4$