What happens when iron is treated with concentrated nitric acid $(HNO_3)$?

The number of $P=O$ and $P-P$ bonds present in the oxoacid of phosphorus,prepared by treating red $P_4$ with alkali,are respectively:

Which of the following molecules contains the highest number of $P-H$ bonds?

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.

In which of the following arrangements,the order is not strictly according to the property written against it?

$PCl_5$ exists but $NCl_5$ does not because