When a calcium atom loses its valence electrons the ion formed has an electron configuration that is the same as an atom of?

Argon. Calcium has the electron configuration 1s2 2s2 2p6 3s2 3p6 4s2, and when it loses its valence electrons (the outermost 4s2 electrons) it has a configuration identical to argon.

If you don't know about sublevels, then disregard that. Calcium has a configuration of 2-8-8-2, and when it loses its 2 valence electrons, it has a configuration identical to argon in the ground state.

Hope that cleared it up