Which of the following could be a cause if there is no beer coming out of the tap?

When there is no beer coming out of the tap, one significant cause could indeed be an empty keg. In a draught system, kegs hold the beer under pressure to ensure it is delivered properly through the lines and out of the faucet. If the keg is empty, there will be no beer available to flow through the tap, leading to the observed issue of no beer coming out. This is a straightforward and common troubleshooting point in bar and brewery settings.

The other options may lead to various problems in beer dispensing systems, but they wouldn't directly result in a complete lack of beer when the tap is engaged. For instance, excessive pressure might lead to beer being over-carbonated or foamy instead of being absent. Incorrect glassware would typically cause issues with beer pouring, such as excessive foam but again would not stop beer from flowing altogether. Lastly, a non-functional cooling system might lead to warm beer or spoilage, but it wouldn't prevent beer from flowing if there is a full keg. Thus, the presence of an empty keg is a clear and direct cause for no beer being dispensed.