Which form of phosphorus is thermodynamically most stable?

On heating,compound $(A)$ gives a gas $(B)$ which is a constituent of air. This gas when treated with $3 \, \text{moles}$ of hydrogen $(H_2)$ in the presence of a catalyst gives another gas $(C)$ which is basic in nature. Gas $(C)$ on further oxidation in moist condition gives a compound $(D)$ which is a part of acid rain. Identify compounds $(A)$ to $(D)$ and also give necessary equations of all the steps involved.

How many $P-O-P$ linkages are there in $P_4O_{10}$?

Which of the following is not correct?

Which one of the orders is correctly matched with the property mentioned against it?

Strong reducing behaviour of $H_3PO_2$ is due to