What is the default volume format for OS X?

The default volume format for OS X is Mac OS Extended (Journaled). This format, often abbreviated as HFS+ (Hierarchical File System Plus), is specifically designed for macOS and offers various features that enhance overall performance and reliability. The inclusion of journaling helps keep track of changes that have not yet been committed to the main part of the disk, which provides a layer of data integrity and faster recovery from crashes or power failures.

In the context of the other choices, FAT32 and exFAT are both file systems compatible with multiple operating systems, including Windows, but they lack some of the advanced features available in Mac OS Extended. For instance, they do not support journaling or certain macOS file permissions that are essential for the system's functionality. NTFS is another format primarily utilized by Windows systems and, with limited read and write support on macOS, is not appropriate as a default for OS X.

Thus, Mac OS Extended (Journaled) is the optimal choice for macOS due to its robust support for the features and functionalities required by the operating system.