Which of the following is an anion?
a. o2-
b. mg2+
c. al3+
d. h

Anions are ions that have a negative charge. Its charge is what we call as valence electrons.
The charge on the ion is written as a superscript after the element symbol. 
You can predict the negative charge on an ion by looking at the group number for the atom.
The charge for representative elements is typically equal to the group minus eight.
Therefore, the anion among the choices is letter A which is .
Oxygen is in group 6A. The negative charge is 6 - 8 = 2. The symbol is  with the charge written as a superscript.