What letter designates the European version of the American keg coupler?

The correct answer is the designation for the European keg coupler system known as the S coupler. This type of coupler is commonly used for European kegs, particularly for those from Germany and some other countries that utilize pressure and valve systems specific to their brewing traditions. The S coupler features a unique configuration tailored to connect seamlessly with the corresponding keg spears.

In contrast, the other coupler types have different applications. The A coupler is primarily used in the United States for certain types of English ales; the D coupler is the most common in the U.S. for domestic beers and certain imports; and the M coupler is specifically designed for use with certain European soft drinks and other beverages. Understanding the specific applications and designs of each coupler type is vital, especially for anyone involved in dispensing draft beer in various contexts.