Which of the following distributions is designed to run directly off a CD drive?

Knoppix is specifically designed to be run directly from a CD (or DVD) without the need for installation onto a hard drive. This makes it a "live" distribution, allowing users to boot into the operating system and use it immediately. Knoppix is known for its ability to provide a fully functional Linux environment, complete with a host of applications, all loaded into memory, which is particularly valuable for troubleshooting and system recovery.

When comparing this to the other distributions, Ubuntu, Arch Linux, and CentOS can all be installed on a system but are not primarily aimed at operating as live CD systems. While they may have live CD versions available, their main use case traditionally involves installation on a hard drive, which is contrary to the design intention of Knoppix. This focus on live functionality for Knoppix is what makes it the correct choice in this context.