Answer:
O2- is an anion
Step-by-step explanation:
An atom or molecule is said to be neutral when it has a zero net charge. In contrast, if the system has a net charge it is termed as an ion.
A cation is an atom or a molecular system with a net positive charge whereas an anion carries a net negative charge.
In the given examples:
Ca2+ : net positive, cation
Ag+: net positive, cation
O2-: net negative, anion
K+: net positive, cation