The flavour of apples is mainly due to which one of the following ?

The correct answer is Option 3: Ethanal.

The distinct and pleasant aroma of apples is a result of a complex mixture of volatile organic compounds. Among these, Ethanal (also known as acetaldehyde) is a primary contributor to the characteristic "green" and fruity flavour profile of fresh apples. While other esters and alcohols contribute to the overall scent, Ethanal is specifically recognized in food chemistry for providing that sharp, fresh apple note.

Regarding other options:

• Formalin is a preservative and disinfectant, highly toxic and not a natural flavouring agent.
• Benzene is a carcinogenic industrial solvent not found as a flavouring component in fruits.
• Benzaldehyde provides a characteristic almond or cherry-like scent, often found in stone fruits (like apricots or peaches) but is not the primary constituent for apple flavour.

Therefore, Ethanal is the most accurate chemical compound responsible for the core flavour of apples among the given choices.