Final answer:
Calcium ions (Ca2+) combine with oxygen ions (O2-) to form the compound calcium oxide (CaO), as they balance each other out in a 1:1 ratio.
Step-by-step explanation:
The compound that forms from Ca2+ and O2- is calcium oxide, which has the formula CaO. This is because calcium ions have a charge of +2 and oxygen ions have a charge of -2. In the compound, the charges must balance, so one Ca2+ ion combines with one O2- ion, and the resulting compound has a neutral charge. No subscripts are necessary in this case because the charges already balance as a 1:1 ratio. Therefore, the correct answer from the provided options is A. CaO.