Diatomaceous earth is a soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It is composed of the fossilized remains of diatoms, a type of hard-shelled algae. Diatomaceous earth finds its application in various fields due to its unique properties. Let's discuss the given options and find out which one is not correct.

Polishing
Diatomaceous earth is used as a mild abrasive in toothpaste, metal polishes, and some facial scrubs. It is also used as a polishing agent in the production of semiconductors and optical components.

Filtration of oils and syrups
Diatomaceous earth is widely used as a filter medium for the clarification of oils, syrups, and other liquids. Due to its high porosity and permeability, it can effectively remove impurities and fine particles from liquids.

Making sound and fireproof rooms
Diatomaceous earth is used as an insulating material in the construction of soundproof and fireproof rooms. It can be mixed with other materials such as cement, plaster, or clay to make lightweight and heat-resistant building blocks.

Biogas production
Diatomaceous earth is not used in biogas production. Biogas is produced by the anaerobic digestion of organic matter such as agricultural waste, sewage sludge, and food waste. Diatomaceous earth has no role in this process.

Conclusion
In conclusion, the correct answer is option D, biogas production. Diatomaceous earth is not used in biogas production, but it finds its application in polishing, filtration, insulation, and other fields.