Phosphorus can be stored in

When $NH_3$ reacts with $Cl_2$,which of the following statements are correct?
$(a)$ $NCl_3$ is formed when $Cl_2$ is in excess.
$(b)$ $N_2$ is formed when $Cl_2$ is in excess.
$(c)$ $N_2$ is formed when $NH_3$ is in excess.
$(d)$ $NCl_3$ is formed when $NH_3$ is in excess.

Which of the following fluorides does not exist?

$Cu +$ conc. $HNO_3$ (hot) $\rightarrow Cu(NO_3)_2 + X$ (oxide of nitrogen);
then $X$ is

Among $P_2O_5$,$As_2O_3$,$Sb_2O_3$,and $Bi_2O_3$,the most acidic oxide is

Which of the following statements is correct regarding $H_3PO_3$ and $H_3PO_4$?