If one of the draft lines is pouring foamy, which of the following could be the cause?

The cause of a draft line pouring foamy often relates to the temperature of the beer in the keg. When beer is served at a temperature that is too high, it increases gas solubility, causing the carbon dioxide in the beer to come out of solution more readily. This results in excessive foam when the beer is poured. A properly chilled keg maintains the right balance of gas and liquid, reducing the likelihood of foaming.

While factors like the speed of opening the faucet, cleaning solutions in the line, or the alcohol content can influence pour quality and stability, the primary factor here is temperature. Ensuring kegs are kept at the optimal serving temperature is essential to minimize issues such as excessive foaming during service.