Saturated solution of  $KN{O}_3$ with agar-agar is used to make ‘salt bridge’ because :

Velocity of $N{O_3}^{-}$   is greater than that of  $K^{+}$

Velocities of  $K^{+}$ and $N{O_3}^{-}$  are nearly the same

Size of $K^{+}$  is greater than that of  $N{O_3}^{-}$

Both velocity and sizes of   $K^{+}$ and $N{O_3}^{-}$  ions are nearly same