Why is nitrogen chemically inert?

The element which catches fire in air at $30^o\,C$ and is stored under water is

In $P_4O_6$,the number of oxygen atoms attached to each phosphorus atom is .......

Assertion $:$ Reducing nature of pnicogen hydrides increases on moving down the group.
Reason $:$ Thermal stability of pnicogen hydrides decreases on moving down the group.

The reaction of $NO$ with $N_2O_4$ at $250 \ K$ gives :

Which one of the following contains a $P-O-P$ bond?