Which of the following is NOT a technique for separating mixtures?

Digestion is the correct response because it is a biological process by which food is broken down into smaller components that can be absorbed into the body, rather than a method for separating mixtures. In contrast, techniques such as filtration, crystallization, and paper chromatography are established laboratory methods specifically designed to separate components of mixtures based on differences in physical or chemical properties.

Filtration involves the use of a barrier (such as filter paper) to separate solids from liquids. Crystallization then allows for the extraction of a solid from a homogeneous solution as it forms pure crystals. Paper chromatography, on the other hand, separates substances based on their movement through a stationary phase (the paper) in the presence of a solvent, allowing for the distinction of different components within a mixture. Thus, digestion is not classified as a separation technique like the others.