Question

Filtration can be used to separate mixtures based on

Answer 1

Filtration is used to separate different particles, so it is based on the particle size.

Answer 2

Filtration can be used to separate mixtures based on the size of their particle.

Filtration is a method to separate different molecule based on size. Filtration mostly used to separate solid substance in a liquid. To separate the molecule, you will need a tools that have smaller holes than the molecule size.
The example of usage of this method would be when you try to remove a substance from homogeneous solution using filter paper. The size of water molecule will be smaller than the paper so the water can pass through. But if the solute size is larger than the paper pore, it will be held and form a residue.