Which of the following compounds gives $N_2$ on heating?

Given below are two statements:
Statement $I$: The halogen that makes the longest bond with hydrogen in $HX$ has the smallest covalent radius in its group.
Statement $II$: $A$ group $15$ element's hydride $EH_3$ has the lowest boiling point among corresponding hydrides of other group $15$ elements. The maximum covalency of that element $E$ is $4$.
In the light of the above statements, choose the correct answer from the options given below.

White phosphorus catches fire in air to produce dense white fumes. This is due to the formation of

Which of the following oxoacids of phosphorus has the strongest reducing property?

Phosphine is prepared by the reaction of

Pure $HNO_3$ is a colorless liquid,but it turns light brown in the presence of light. This is due to its decomposition into which of the following substances in small amounts?