To determine this simply multiply the number of oxygen atoms by the oxidation number for each oxygen atom, which is 4 • -2 = -8. Then knowing we have 1 Mn atom, it would be 1 • x, where x is the unknown oxidation state for Mn.
X + - 8 = overall charge of compound
X + - 8 = -1
X - 8 = -1
X = 7.
The solution is C.