What system is used to keep the keg lines cold?

The correct answer is glycol because this system is specifically designed to maintain the temperature of the beer as it travels through the keg lines to the tap. Glycol is a coolant that is circulated alongside the beer lines in insulated tubing, ensuring that the beer remains cold from the keg to the point of service. This method is particularly effective in minimizing foam and providing a consistent serving temperature, which is crucial for delivering the best taste and quality of the beer.

Refrigeration units do play a vital role in keeping kegs cold when they are stored, but they do not specifically address the need to maintain the temperature of the beer as it moves through the lines. Air conditioning generally serves to cool larger areas and is not suited for the specific needs of keg line temperature control. Coolant fluid, while it may sound relevant, does not accurately describe the specific method used in these systems, as glycol refers specifically to the type of coolant utilized in such contexts. Thus, glycol's unique properties and application make it the correct answer for maintaining optimal temperatures in keg lines.