In which pair do both atoms have one electron only in an s orbital in their ground states?

a ca, sc b cu, be c h, he d li, cr

what does electron in ground states means?
and what is the possible answer?

The answer to this is letter D.

Chromium's electron configuration is 1s2 2s2 2p6 3s2 3p6 3d5 4s1, where the last electron is in the s orbital with only 1. As for Li, it is found in the group 1 metals, and it is a fact that all elements under this group has the their last electron to be in the s orbital with only one electron in their outermost shell. 

The electron configuration associated with the lowest energy level of the atom is referred to as ground state and each electron ion is in the lowest energy level possible. When the moment comes that electrons go into a higher level of energy, that is what we call the excited state. 

amount of time the two substances are in contact. area in contact between the two substances. specific heat of the material that makes up the substances. the density of the two substances in contact.