What is the oxidation state of S in $\mathrm{SO}_4^{2-}$ ?

Which compound from following contains iodine with highest oxidation number?

In which of the following compounds chlorine has highest oxidation state?

Identify the element reduced in following reaction.

$$\mathrm{Cr}_2 \mathrm{O}_7^{2-}+14 \mathrm{H}^{+}+6 \mathrm{I}^{-} \longrightarrow 2 \mathrm{Cr}^{3+}+3 \mathrm{H}_2 \mathrm{O}+3 \mathrm{I}_2$$