Explanation

(c)
The chemical structure of $H_{2}S_{2}O_{8}$ is as follows
O O
|| ||
$H - o - s - o - o - s - o - H$
||$\$ ||
O O
$2 \times ( + 1) + 2 \times x + 6 \times ( - 2) + 2 \times ( - 1) = \ 0$
^{for H for S for O for O}$-$^{O}
+2$+ 2x - 12 - 2 = 0$
$2x = \ + 12$
$x = \ + 6$