Which of the following is a cause of a failure to extract?

A worn extractor can lead to a failure to extract because the extractor is responsible for gripping the cartridge case as the slide moves rearward. When the extractor is worn or damaged, it may not properly engage the rim of the cartridge, making it ineffective at pulling the spent casing out of the chamber. This can result in malfunctions during the cycling of the firearm, where the casing remains lodged in the chamber instead of being ejected.

While other factors such as a dirty barrel, a weak hammer spring, and over-lubing parts can cause various issues in firearm operation, they do not specifically target the extracting mechanism as directly as a worn extractor does. For instance, a dirty barrel can affect accuracy and the reliability of firing, but it does not directly hinder the extraction process. Similarly, a weak hammer spring may lead to failure to ignite the primer rather than impacting the extraction. Over-lubing parts might create issues with functionality but wouldn’t specifically obstruct the extractor's capability to pull a shell from the chamber. Thus, the focus on the extractor being worn directly correlates with the failure to extract, making it the correct choice in this scenario.