Answer: particle size
Explanation:
1. Density is defined as mass contained per unit volume.
If two substances have different densities, the one with higher density sinks in the one with lower density.
2. Magnetism is the property of an object that can attract or repel objects that have a magnetic material like iron inside them.
It is used in separation when one of the material is magnetic.
3. Boiling point is the temperature when liquid converts to gaseous form.
It is used to separate two liquids with different boiling points using distillation.
4. Particle size is used to separate two solids with different sizes. Sand particles are smaller than seashells and thus can be separated using sieve.