What directory is always identified by the slash symbol (/) and is in the "a" partition?

The directory identified by the slash symbol (/) is known as the root directory. In UNIX and UNIX-like operating systems, the root directory is the top-level directory in the file system hierarchy. It serves as the starting point from which all other directories are organized; therefore, every file and directory is accessible from the root directory by traversing through various subdirectories.

When a computer system is partitioned, the root directory is essential to access all other partitions, as it contains subdirectories that point to the locations of other files and directories, including those in other partitions like the "a" partition referenced in the question. Understanding that the root directory is universally represented by the slash symbol helps in navigating and managing files across the filesystem.

The other options listed refer to various types of directories but do not possess the unique identifier of the root directory in the context provided. For instance, user, binary, and device directories are vital for specific functions but do not represent the top level of the directory structure.