Carbon is in the lowest oxidation state in

Consider the following compounds:
$\underline{K}O_2$,$H_2\underline{O}_2$ and $H_2\underline{S}O_4$.
The oxidation states of the underlined elements in them are,respectively,

Oxidation number of $N$ in $(NH_4)_2SO_4$ is

What is the oxidation state of sulphur in oil of vitriol?

What is the oxidation number of underlined species in $\underline{P}F_6^{-}$ and $\underline{V}_2 O_7^{4-}$ ions respectively?

The oxidation number of the underlined atom in the following species is given. Identify the incorrect option.