Which of the following is not heterogeneous
An Emulsion
True solution
Suspension
Colloidal sol
True Solution is a homogeneous mixture of two or more substances in which substance dissolved (solute) in solvent has a particle size of less than 10-9 m or 1 nm. The simple solution of sugar in water is an example of the true solution. Particles of true solution cannot be filtered through filter paper and are not visible to the naked eye.
A colloidal solution is heterogeneous.
Emulsion and suspension are also heterogeneous.
So, option 2 is correct.