Final answer:
In single user mode for Unix-like operating systems, the only essential partition that is mounted is the root partition, which is necessary for system maintenance and recovery.
Step-by-step explanation:
When a Unix-like operating system boots into single user mode, it mounts only the essential filesystems needed for basic system maintenance. In this mode, the most crucial partition that is mounted is the root partition. This is necessary because the root partition contains critical system files and utilities used for repairing the system, performing maintenance tasks, or recovering from system issues. The boot and usr partitions are not typically mounted in single user mode by default because they contain less critical system files and user-installed applications, which are not needed for these core administrative tasks.