Complete the electron configuration of a calcium atom by selecting the correct orbitals.
a) 2s2 2p6 3s2 3p6 4s2
b) 1s2 2s2 2p6 3s2 3p6
c) 2s2 2p6 3s2 3p6 4s1
d) 1s2 2s2 2p6 3s2 3p6 4s2