Which drink mixer is represented by the letter "D" on the soda gun?

The drink mixer represented by the letter "D" on a soda gun is typically Diet Coke. Soda guns are commonly used in bars and restaurants to dispense various soft drinks and mixers efficiently. Each letter or symbol on the gun corresponds to a specific beverage.

Diet Coke is a popular choice in many establishments, and it fits the typical labeling system where drinks are often abbreviated by their initial letters.

While Dr. Pepper is also a widely consumed soft drink, it does not typically share the same designation as Diet Coke on a soda gun, since it may have a different letter representation depending on the setup of the establishment. Daiquiri Mix and Dark Rum, being a cocktail mixer and a spirit respectively, would not be designated by a single letter "D" in the context of a soda gun, which primarily features carbonated soft drinks. Thus, the selection of Diet Coke is consistent with common bartending practices.