What is the oxidation state of $Fe$ in $Fe_3O_4$?

What is the oxidation state of nitrogen in the oxide of nitrogen formed by heating ammonium nitrate?

The oxidation number of $N$ in $NH_3$ is:

One mole of $N_2H_4$ loses $10 \ moles$ of electrons to form a new compound $Z$. Assuming that all the nitrogen atoms appear in the new compound, what is the oxidation state of nitrogen in $Z$? (There is no change in the oxidation state of hydrogen.)

Iodine reacts with concentrated $HNO_3$ to yield $Y$ along with other products. The oxidation state of iodine in $Y$ is

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