Find the total number of reagents 'M'

$\text{ (conc. }\left.{\mathrm{HNO}}_3\right),{\mathrm{O}}_2,{\mathrm{H}}_2{\mathrm{O}}_2,{\mathrm{N}}_2,{\mathrm{NH}}_3,\left({\mathrm{Br}}_2+{\mathrm{H}}_2\mathrm{O}\right)$

The reagent of the M in the complete reaction can be,${\mathrm{HNO}}_3,{\mathrm{O}}_2,{\mathrm{H}}_2{\mathrm{O}}_2,\left({\mathrm{Br}}_2+{\mathrm{H}}_2\mathrm{O}\right)$ 

these are 4 in number. thus, The correct answer is 4.