Which of the following represents the correct decreasing order of size of the particles of solution, suspension and colloid? 

The correct decreasing order of size of the particles of solution, suspension and colloid is: Suspension $>$ Colloid $>$ True solution.

A true solution is a homogeneous mixture. The size of the particle of a true solution is very small i.e., less than $1\mathit{nm}$. 

Suspension and colloids are heterogeneous mixtures. The particle size of the colloid varies between $1\mathit{ nm}$ to $100\mathit{ nm}$ and they are relatively larger than particles of a true solution. 

In the case of suspension, the particle size is large i.e., greater than $100\mathit{ nm}$ and they scatter a beam of light passing through it.