Which of the following materials is used to represent the sour taste?

Citric acid effectively represents the sour taste because it is a well-known organic acid commonly found in various fruits, particularly citrus fruits like lemons and limes. The sourness associated with citric acid arises from its ability to release hydrogen ions in solution, which stimulates the sour taste receptors on the tongue. This chemical property makes citric acid a prime example of what gives foods their sour flavor, setting it apart from the other options listed. Sucrose, being a sugar, tends to impart sweetness instead; monosodium glutamate is primarily associated with umami (savory) flavor; and caffeine adds bitterness, distinguishing citric acid as the correct choice for representing sour taste.