ANS:D - carbon dioxide.

The commonly used solvent in supercritical extraction is carbon dioxide (CO2). Supercritical extraction, also known as supercritical fluid extraction (SFE), is a process used to extract compounds from solid or liquid materials using a supercritical fluid as the solvent. In supercritical extraction, carbon dioxide is the most widely used solvent due to its favorable properties:

1. Supercritical State: Carbon dioxide becomes supercritical at relatively low temperatures and moderate pressures, making it easy to work with and manipulate within the supercritical region.
2. Non-Toxic and Inert: Carbon dioxide is non-toxic, non-flammable, and environmentally friendly. It does not leave behind any residue in the extracted products, making it suitable for use in food, pharmaceutical, and cosmetic industries.
3. Tunable Solvency: The solvency power of carbon dioxide can be adjusted by changing the temperature and pressure conditions, allowing for selective extraction of target compounds while leaving unwanted components behind.
4. Easy Separation: After extraction, carbon dioxide can be easily separated from the extracted compounds by simple depressurization, leaving behind a clean, solvent-free product.

Methyl ethyl ketone and carbon tetrachloride are not commonly used as solvents in supercritical extraction due to their toxicity and environmental concerns. While water can be used as a solvent in certain supercritical extraction processes, carbon dioxide is more commonly employed due to its unique properties and versatility.