Final answer:
CH₃CH₂COOH (option C) is an acid as it contains the carboxylic acid group -COOH, which can dissociate to release H+ and show acidic behavior.
Step-by-step explanation:
The student asked which of the following is an acid: A. CH₃CH₂OH, B. CH₃CH₂COCH₃, C. CH₃CH₂COOH, D. CH₃CH₂COOCH₃. Among these compounds, CH₃CH₂COOH is an acid. Specifically, it is a type of organic compound known as a carboxylic acid. Carboxylic acids have the functional group -COOH, which can dissociate in water to release an H+ ion (a proton), thus behaving as an acid. A molecule like CH₃CH₂COOH can lose an H+ from the -COOH group, forming CH₃CH₂COO- and a H+ in solution, showing its acidic characteristics.