how is

True solution stable

Colloid is stable

and Suspension is unstable.

In a True solution the solute particles are very small (less than 1nm) so they easily mix in the solvent.

Colloid particles are also small but larger than true solution(1nm to 100nm) so they also mix and do not settle at the bottom.

But in suspension the particles are quite large (larger than 100nm) so they do not mix and settle at the bottom so they are unstable.