The question asks how very small insoluble particles in a liquid can be separated from it.

Option 1: Crystallization is not the appropriate method for separating small insoluble particles. Crystallization is the process of separating a solute from a solvent by allowing the solvent to evaporate and form crystals of the solute.

Option 2: Fractional distillation is used for separating two or more liquids with different boiling points. It is not suitable for separating small insoluble particles.

Option 3: Centrifugation is the process of using centrifugal force to separate substances of different densities. This method is effective for separating small insoluble particles from a liquid. When a mixture is spun rapidly in a centrifuge, the heavier particles settle at the bottom, allowing the liquid to be decanted or poured off.

Option 4: Decantation is a process where a liquid containing sediment or insoluble particles is slowly poured off, leaving the particles behind. While it may be effective for larger particles, it may not be as effective for very small insoluble particles.

The correct answer is option 3, centrifugation, as it is the most suitable method for separating very small insoluble particles from a liquid.