Which data structure is primarily used for indexing files on a hard disk?

The tree data structure is primarily used for indexing files on a hard disk because it allows for efficient searching, insertion, and deletion of file records. Specifically, a type of tree called a B-tree, or its variations like B+ trees, is commonly utilized in file systems for indexing purposes.

These tree structures facilitate quick access to large amounts of data due to their balanced nature, which ensures that all leaf nodes are at the same depth, thereby providing a predictable performance when searching for files. The hierarchical organization of data in a tree makes it easier to manage and navigate through the files, especially when dealing with large volumes of information spread across disk segments.

In contrast, linked lists and arrays do not offer the same efficiency in searching or sorting data, making them less suitable for indexing operations where quick access to a file is essential. Hash tables, while useful for quick lookups based on key values, do not support ordered data retrieval or hierarchical organization, which are critical for file indexing systems. Thus, the use of tree structures aligns perfectly with the requirements of file indexing on hard disks.