Each oxygen will have the oxidation number (2-), and we know the total charge is 1-. We have four oxygen atoms per molecule, so we can set up the equation:
(charge on Mn) + 4(charge on O) = -1
(charge on Mn) + 4(-2) = -1
(charge on Mn) - 8 = -1
Charge on Mn = 7
The answer is C. 7
