Which of the following elements exhibits a valency of $2$?

The oxidation state of $+1$ for phosphorus is found in:

Predict the formulas of the stable binary compounds that would be formed by the combination of the following pairs of elements.
$(a)$ Lithium and oxygen
$(b)$ Magnesium and nitrogen
$(c)$ Aluminium and iodine
$(d)$ Silicon and oxygen
$(e)$ Phosphorus and fluorine
$(f)$ Element $71$ and fluorine

$A$ compound contains three elements $A$,$B$ and $C$. If the oxidation states are $A = +2$,$B = +5$ and $C = -2$,the possible formula of the compound is

Select the $CORRECT$ order of stability.

The valency (oxidation state) of phosphorus in $H_3PO_4$ is