Find the number of reagents in which ${\mathrm{Ag}}_2\mathrm{S}$ is insoluble,

$\text{ hot conc. }{\mathrm{HNO}}_3; {\left({\mathrm{NH}}_4\right)}_2\mathrm{S}; {\mathrm{NH}}_3; \mathrm{KCN}; {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$

Reagents in which Silver sulphide ${\mathrm{Ag}}_2\mathrm{S}$ is insoluble are

${\left({\mathrm{NH}}_4\right)}_2\mathrm{S};{ \mathrm{NH}}_3; \mathrm{KCN}; {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$

Thus, the correct answer is 4.