Which of the following alcohols is a primary alcohol?

A primary alcohol is defined as an alcohol where the hydroxyl group (-OH) is attached to a carbon atom that is bonded to only one other carbon atom, making it a terminal carbon. In the case of butanol, propanol, and ethanol, each of these alcohols qualifies as primary:

• Butanol has a four-carbon chain with the hydroxyl group on the first carbon, indicating that it is a primary alcohol.

• Propanol features a three-carbon chain, and when the hydroxyl group is attached to the first carbon, it is also classified as primary. Note that there may be isomers, but the most common structure we consider is 1-propanol.

• Ethanol, with a two-carbon chain, has its hydroxyl group on the first (and only) carbon, making it primary as well.

Since all three examples—butanol, propanol, and ethanol—fit the definition of primary alcohols based on their structures, the correct answer is that all of the above are primary alcohols.