(c)
The oxidation state of iodine in $HIO_{4}\ is + 7$ as
$1 + x + 4( - 2) = 0$
$x = + 7$
The oxidation state of iodine in $H_{3}{IO}_{5}$ is +7 as
3$+ x + 5( - 2) = 0$
$x = + 7$
The oxidation state of iodine in $H_{5}IO_{6}$ is +7 as
5$+ x + 6( - 2) = 0$
$x = + 7$