A hard disk is divided into tracks which are further subdivided into:

A hard disk is divided into tracks, which are further subdivided into sectors.
Sectors are the smallest unit of storage on a hard disk and typically contain 512 bytes or 4 kilobytes of data.
When data is written to or read from a hard disk, it is done at the sector level.
Clusters are groups of sectors, used by the file system to manage and allocate disk space, but they are not the fundamental subdivision of tracks.
Vectors and heads are not subdivisions used within the structure of a hard disk.
Therefore, the correct option is sectors.