In true solution, size of solute particles is 

A solution is defined as a homogeneous mixture containing two or more substances, a solute is a substance that dissolves, and a solvent is the liquid portion of a solution that mixes and dissolves.

The light rays cannot be scattered, the particles of the solution cannot be seen with the naked eye, and the solute particles cannot be separated from the solution by filtration, it contains only one phase. 

Solutions can be divided into three types, true solutions, colloidal solutions, and suspensions, based on particle size. 

A true solution has a particle size of fewer than 0.001 micrometers or 1 nanometer and is defined as a homogeneous mixture of substances that are invisible to the naked eye. 

A colloidal solution is a solution that produces a heterogeneous mixture of particles. Particle sizes range from 1 to 1000 nanometers, and like real solutions, particles are invisible to the naked eye.

The suspension is also a heterogeneous mixture, the particle size is over 1000 nanometers, and the particles are large enough to be seen with the naked eye.

 Therefore, the correct option is (A) In a true solution, the size of solute particles is  0.001 micron.