Which orbital (or ring) contains the electron that calcium loses to form an ion?

4s orbital contains the electron that calcium loses to form an ion

Explanation:

Electronic configuration represents the total number of electrons that a neutral element contains.  The electrons are filled according to Afbau's rule in order of increasing energies.

Calcium with atomic number 20 has 20 electrons and has 2 valence electrons which it can lose to for stable ion. The valence elctrons are present in valence shell i.e. 4s.