Final answer:
The oxide ion (O2-) is an anion because it carries a negative charge, while the other ions listed are cations with positive charges.
Step-by-step explanation:
An anion is a negatively charged ion, which means it has gained electrons. In the options provided, (a) O2- represents an anion, specifically the oxide anion. This is because it carries a negative charge, indicated by the '2-' after the 'O' for oxygen. The other options like Ca2+, K+, and Ag+ represent cations, which are positively charged ions.