Which of the following describes a malfunction of a revolver?

A failure to extract occurs when a round that has been fired does not get pulled out of the chamber during the cycling of the revolver's action. This can happen for several reasons, such as the spent cartridge case becoming stuck due to dirt, damage, or improper ammunition. In a revolver, this malfunction prevents the firearm from rotating to the next chamber, rendering it unable to fire again until the issue is resolved. This type of malfunction specifically highlights a problem with the extraction process which is vital for the operation of a revolver.

Other malfunctions mentioned, while also significant, relate to different aspects of the firearm's operation. For instance, a failure to load refers to issues that prevent a cartridge from being placed into the cylinder, a double feed typically occurs in semi-automatic actions where two rounds attempt to enter the chamber simultaneously, and a failure to eject indicates the spent casing is not being expelled from the cylinder after firing. Each of these descriptors points to different mechanisms within a firearm's operation, but the failure to extract is distinctly related to the incomplete removal of an already fired round, making it a unique malfunction indicative of specific issues with the revolver.