Which of the following answers below is the correctly balanced half-reaction for the reduction of the SO4 ^-2 ion in acid solution? The answer is C, but I don’t understand why. Thanks in advance :-)

C

Explanation:

The oxidation number of Sulphur in SO4^2- is;

x + 4(-2) = -2

x - 8 = -2

x = -2 + 8

x = 6

Now,

the oxidation number of sulphur in H2SO3 is

2 (1) + x + 3(-2) = 0

2 + x -6 = 0

-4 + x = 0

x = 4

Hence, the oxidation number of sulphur changed from +6 to +4 which signifies  gain of two electrons as shown in option C.