The question asks for a useful separation process in the isolation of the components of plant material. Sieving is used to separate solids of different particle sizes. It is not suitable for isolating chemical components from plant extracts. Sublimation is used to separate a solid that sublimes from one that does not. While some plant compounds might sublime, it's not a general method for isolating the diverse components of plant material. Chromatography is a highly versatile technique used to separate components of a mixture based on their differential affinities for a stationary phase and a mobile phase. It is widely used in natural product chemistry to isolate and purify various compounds (e.g., alkaloids, flavonoids, terpenes) from plant extracts. Distillation is used to separate liquids with different boiling points or to separate a liquid from non-volatile solids. It can be used for volatile components like essential oils, but many important plant compounds are non-volatile or thermally sensitive, making distillation unsuitable for their isolation. Among the given options, chromatography is the most effective and widely used method for isolating a broad range of components from complex plant materials. The correct option is (C). The final answer is $\boxed{\text{C}}$. That's 2 down. 3 left today — send the next one.