The question asks about the main factor responsible for the flavor of apples. It is important to note that while all the options mentioned are chemical compounds, only one of them is responsible for the flavor of apples.

Option 1: Formalin - Formalin is a solution of formaldehyde, and it is not responsible for the flavor of apples. This option can be eliminated as it is not relevant to the question.

Option 2: Benzene - Benzene is a chemical compound that is not found naturally in apples. Therefore, it is not the main factor responsible for the flavor of apples.

Option 3: Ethanal - Ethanal, also known as acetaldehyde, is an organic compound that is responsible for the fruity aroma and flavor in apples. This option is correct as it directly addresses the main factor responsible for the flavor of apples.

Option 4: Benzaldehyde - Benzaldehyde is a chemical compound that is responsible for the almond-like aroma and flavor. While it may contribute to the overall flavor profile of some apple varieties, it is not the main factor responsible for the flavor of apples.

In conclusion, the correct answer is option 3, ethanal, as it is the main factor responsible for the flavor of apples.