Which structure of the HFS volume keeps track of allocations of blocks in use and those that are free?

The structure that keeps track of allocations of blocks in use and those that are free within an HFS (Hierarchical File System) volume is the volume bitmap. This bitmap is crucial for managing storage space on the volume, detailing which blocks are occupied by files and which ones are available for new data.

Using a volume bitmap allows the file system to efficiently allocate space when new files are created or existing files are modified. It essentially acts as a visual representation of the disk's used and free space, which helps in optimizing data storage management. The volume bitmap integrates seamlessly with file operations, ensuring that data can be written to available spaces without overlap or corruption.

The other choices refer to different aspects of file systems. The allocation table might imply a structure used for tracking allocations, but it's not specific to HFS or defined as such; it’s more generic and can vary across different file systems. Logical block 3 is a specific block in the structure of the HFS volume but does not serve the purpose of tracking allocations. The file descriptor table is primarily used to keep track of file-related metadata, such as file pointers and access permissions, rather than the status of blocks. Thus, the correct answer directly corresponds to the specific mechanism used in HFS for