Which of the following could cause a failure to fire in a shotgun?

In a shotgun, having the safety engaged prevents the trigger from being pulled, thereby blocking any chance of firing a round. This is a critical safety feature designed to keep the firearm from discharging unintentionally. When the safety is in the "on" position, the mechanical components ensure that even if the trigger is pulled, the firing mechanism remains inactive.

The other options relate to different potential issues that can arise but do not inherently cause a failure to fire in the same straightforward manner. For example, if the chamber is too full, it may lead to jamming or other feeding issues, but that situation wouldn’t directly impede the act of firing if the shotgun is functioning correctly. If the round is fired, it obviously indicates the shotgun has fired, so it would not be relevant to a failure to fire scenario. Lastly, a cracked stock does not necessarily prevent the firing mechanism from engaging; it can affect the overall integrity and handling of the shotgun but does not directly lead to a failure to fire.