The electronic configuration of $\ce{Ca}$ ($Z = 20$) is:

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