Which of the following reagents can be used for making the distinction between $\mathrm{AgCl}$ and $\mathrm{AgI}$? 

Here,

$\mathrm{AgCl}\stackrel{{\mathrm{AsO}}_3^{3-}}{⟶}\stackrel{{\mathrm{Ag}}_3{\mathrm{AsO}}_3\downarrow }{(\mathrm{yellow} \mathrm{ppt}.)}$

Agl does not react.

$\mathrm{AgCl}+{\mathrm{NH}}_3⟶\stackrel{\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]\mathrm{Cl}}{\left(\mathrm{Soluble}\right)}$

AgI does not dissolve.

Thus, a and b are correct.