An oxide of nitrogen (X) is formed when Z is reacted with $P_2O_5$. X is soluble in water and gives Z. Which one of the following is Z?

Question

$\begin {array} {1 1} (1)\;H_2N_2O_2 & \quad (2)\;HN_3 \\ (3)\;HNO_2 & \quad (4)\;HNO_3 \end {array}$