What is the structure of acetaldehyde?

The structure of acetaldehyde is best represented as CH₃-C=O. Acetaldehyde is an aldehyde with the molecular formula C₂H₄O, characterized by the presence of a carbonyl group (C=O) located at the end of its carbon chain.

The correct depiction shows a three-carbon chain where the carbonyl group is directly bonded to one of the terminal carbons (which is also bonded to a hydrogen atom) making it an aldehyde. This structure is consistent with the definition of aldehydes, which must have the carbonyl group at the end of the carbon chain (as opposed to ketones, where it's found within the carbon chain).

In contrast, the other choices do not correctly depict the structure of acetaldehyde. For instance, the structure in 'B' suggests that there is a hydroxyl group (OH) directly involved in the carbonyl context, which does not align with the aldehyde structure. The option 'C' presents a hydroxyl attachment without an appropriate carbon skeleton for an aldehyde, while 'D' confuses the carbonyl and carbon chain placements. Hence, those options do not fulfill the requirements for acetaldehyde's structure.