What does the directory /run/udev/rules.d contain?

The directory /run/udev/rules.d is designed to hold udev's volatile rules, which are generated at runtime. These rules are used by the udev device manager to determine how to handle devices when they are added to or removed from the system. The contents of this directory can be used for dynamically managing device nodes and their settings, allowing for quick adjustments based on the current system state.

In contrast, other directories such as /etc/udev/rules.d contain persistent udev rules that are defined by the system administrator and persist across reboots. Therefore, while the other options describe different types of udev rules or related concepts, the volatile nature and runtime generation of the rules present in /run/udev/rules.d distinctly identifies it as the correct answer regarding its contents.