How do you export a Docker container's filesystem to a tar archive?

Exporting a Docker container's filesystem to a tar archive is accomplished using the command specifically designed for this purpose, which is the export command. This command allows you to take the entire filesystem of a specific running or stopped container identified by its container ID and create a tar archive of that filesystem. The resulting tar file can be used for backup purposes, transferring the container's filesystem, or for any other reasons where a snapshot of the container's state is required.

Using the export command collects everything in the container's filesystem and puts it into the tar archive, allowing for easy portability and storage. Other commands listed do not perform this specific task. For instance, saving a Docker image with the save command is aimed at exporting images, not containers, while the archive and pack commands do not exist in the context of typical Docker CLI usage.

Thus, option B correctly outlines the proper command to achieve the desired outcome of exporting a container’s filesystem.