In the diagram above, the mixture of the solids P and Q can be separated by

The diagram shows two solids, P and Q, mixed together. The question is asking how these two solids can be separated from each other. Crystallization involves dissolving a solid in a solvent to create a solution, then allowing the solvent to evaporate slowly to produce crystals of the dissolved solid. This method can be used when one of the solids is soluble in a particular solvent and the other is not. However, from the diagram, it is not clear whether either of the solids is soluble in a particular solvent, so we can eliminate options C and D. Distillation is a process used to separate liquids based on differences in their boiling points. This method would not be effective for separating solids, so we can eliminate option A. Fractional distillation is similar to distillation, but is used to separate a mixture of liquids with similar boiling points. This method also would not be effective for separating solids, so we can eliminate option B. Therefore, the correct answer is option C, crystallization.