Explanation
The /var directory must be mounted with read and write access if it resides on its own dedicated file system. The reason is that the /var directory contains files and directories that are expected to change in size and content as the normal operation of the system progresses, such as logs, spool files, and temporary files1.
Therefore, the /var directory needs to have enough space and permission to accommodate these changes. If the
/var directory is mounted as read-only, some system services and applications may fail to start or function properly2.
The other options are not directories that must be mounted with read and write access if they reside on their own dedicated file system. The /opt directory contains optional or third-party software packages that are not part of the default installation1. The /lib directory contains libraries and kernel modules that are essential for the binaries in /bin and /sbin directories1. The /etc directory contains configuration files for the system and applications1. The /usr directory contains user-related programs, libraries, documentation, and data1. These directories are usually mounted as read-only to prevent accidental or malicious modification of their contents3. References:
* Linux Essentials Exam Objectives, Version 1.6, Topic 102.1, Weight 3
* Linux Essentials Certification Guide, Chapter 2, Page 34-35
* Linux Filesystem Hierarchy, Chapter 3, Page 17-18