Identify an aldehyde used in margarine and in food for its buttery odour.

The correct answer is (2) Butyraldehyde.
Butyraldehyde (also known as butanal) is an aldehyde that has a characteristic buttery odor. It is commonly used in the food industry to impart a buttery flavor, and it is also found in margarine to enhance its aroma.
Here's a quick look at the other options:
- (1) Benzaldehyde: This aldehyde has an almond-like aroma and is used in flavorings and perfumes, but not for a buttery odor.
- (3) Cinnamaldehyde: This aldehyde is responsible for the characteristic flavor and aroma of cinnamon, not butter.
- (4) Oxaldehyde: This is not a common aldehyde and does not fit the description related to buttery odors.
Thus, Butyraldehyde is the aldehyde associated with the buttery scent in margarine and other food products.