A tiny sphere of mass m and density x is dropped in a tall jar of glycerine of density y . When the sphere acquires terminal velocity, the magnitude of the viscous force acting on it is 

When the sphere acquires terminal velocity, the upward viscous force equals the apparent weight in glycerine. Therefore, the magnitude of the viscous force is (here r is the radius of the sphere) 

           $F=\frac{4}{3}{\mathrm{\pi r}}^3\left(\mathrm{x}-\mathrm{y}\right)\mathrm{g}=\frac{\mathrm{m}}{\mathrm{x}}\left(\mathrm{x}-\mathrm{y}\right)\mathrm{g}=\mathrm{mg}\left(1-\frac{\mathrm{y}}{\mathrm{x}}\right)$

Hence the correct choice is (3).