Explanation
The fastest way to boot the Linux system to normal mode is to comment out the /opt/app filesystem in
/etc/fstab and reboot. This will prevent the system from trying to mount the /opt/app filesystem at boot time, which causes an error because the filesystem does not exist or is corrupted. Commenting out a line in /etc/fstab can be done by adding a # symbol at the beginning of the line. Rebooting the system will apply the changes and allow the system to boot normally. Reformatting the /opt/app filesystem will not help to boot the system, as it will erase any data on the filesystem and require manual intervention to create a new filesystem.
Performing filesystem checks on local filesystems will not help to boot the system, as it will not fix the missing or corrupted /opt/app filesystem. Triggering a filesystem relabel will not help to boot the system, as it will only change the security context of files and directories according to SELinux policy. References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 14: Managing Disk Storage, page 456.

Explanation
The command docker images prune -a will help to remove all dangling images and delete all the images that do not have an associated container. The docker command is a tool for managing Docker containers and images. The images subcommand operates on images. The prune option removes unused images.
The -a option removes all images, not just dangling ones. A dangling image is an image that is not tagged and is not referenced by any container. This command will accomplish the task of cleaning up the unused images.
The other options are incorrect because they either do not exist (docker push images -a or docker images rmi
--all) or do not remove images (docker rmi -a images only removes images that match the name or ID of
"images"). References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 19: Managing Cloud and Virtualization Technologies, page 567.

Explanation
The command that can be used to check for running containers is docker ps. The docker ps command can list all the containers that are currently running on the system. To show all the containers, including those that are stopped, the administrator can use docker ps -a .
[CompTIA Linux+ Study Guide], Chapter 11: Working with Containers, Section: Managing Containers with Docker
[Docker PS Command with Examples]

The server is in a "Listen" state on port 9943 using its loopback address. The "1234" is a process-id

Explanation
The command iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination
192.0.2.25:3128 adds a rule to the nat table that redirects all incoming TCP packets with destination port 80 (HTTP) to the proxy server 192.0.2.25 on port 3128. This is the correct way to achieve the task. The other options are incorrect because they either delete a rule (-D), use the wrong protocol (top instead of tcp), or use the wrong port (81 instead of 80). References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 12: Managing Network Connections, page 381.