For a useful separation process in the isolation of the components of plant material.

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 $\boxed{\text{C}}$.