In which compound is the oxidation state of phosphorus $+4$?

What is the oxidation number of carbon in $K_2C_2O_4$?

Assign the oxidation number to the underlined element in each of the following species:
$1$. $NaH_{2}\underline{P}O_{4}$
$2$. $NaH\underline{S}O_{4}$
$3$. $H_{4}\underline{P_{2}}O_{7}$

What is the oxidation number of sulphur in $Na_2S_4O_6$?

The oxidation states of $P$ atom in $POCl_3$,$H_3PO_3$ and $H_4P_2O_6$,respectively are

Write the oxidation number of oxygen in $KO_{3}$ and $Na_{2}O_{2}$.