Which one of the following combines with $Fe(II)$ ions to form a brown complex?

$A$ pungent smelling gas $A$ gives dense white fumes with conc. $HCl$. When $A$ reacts with an alkaline solution of a colourless reagent $B$,a reddish-brown precipitate $C$ is formed. The compounds $A, B, C$ respectively are:

Total number of acidic oxides among $N_2O_3, NO_2, N_2O, Cl_2O_7, SO_2, CO, CaO, Na_2O$ and $NO$ is $...........$

Red phosphorus can be obtained from white phosphorus by:

During the borax bead test with $CuSO_4$,a blue-green colour of the bead was observed in the oxidising flame due to the formation of:

The most stable trihalide of nitrogen is ..... .