Flavouring agents are substances that are added to food and beverages to enhance their taste and aroma. They provide a specific flavor profile and are used in various forms such as natural extracts, essential oils, or synthetic compounds. Among the options given, acetophenone (option D) is commonly used as a flavouring agent. Let's discuss the properties and applications of acetophenone in detail.

Properties of Acetophenone:
- Acetophenone is a colorless liquid with a sweet, flowery aroma.
- Its chemical formula is C8H8O, and it belongs to the class of organic compounds known as ketones.
- Acetophenone has a boiling point of around 202 °C and is slightly soluble in water.

Applications of Acetophenone as a Flavouring Agent:
- Acetophenone is commonly used in the food and beverage industry as a flavouring agent due to its pleasant aroma and taste.
- It is often used to add a sweet, fruity, or floral note to various products, including baked goods, candies, ice creams, beverages, and chewing gums.
- Acetophenone is also used in the production of artificial vanilla flavoring.
- In addition to its use as a flavouring agent, acetophenone is utilized in the synthesis of pharmaceuticals, fragrances, and dyes.

Other Options Explained:
a) Methanal (formaldehyde) is a pungent gas and is not used as a flavouring agent.

b) Ethanol (ethyl alcohol) is primarily used as a solvent and preservative in the food and beverage industry, but it is not commonly used as a flavouring agent.

c) Acetone is a solvent and is primarily used in industrial applications such as paint thinners and nail polish removers. It is not used as a flavouring agent.

To summarize, among the given options, acetophenone (option D) is the correct answer as it is commonly used as a flavouring agent in the food and beverage industry. Its sweet, flowery aroma makes it suitable for adding pleasant flavors to a variety of products.