The command echo 'export PATH=$PATH:/opt/operations1/bin' >> /etc/profile should be used to resolve the issue of users not being able to access the application without using the full path. The echo command prints the given string to the standard output. The export command sets an environment variable and makes it available to all child processes. The PATH variable contains a list of directories where the shell looks for executable files. The $PATH expands to the current value of the PATH variable. The : separates the directories in the list. The /opt/operations1/bin is the directory where the application is installed.
The >> operator appends the output to the end of the file. The /etc/profile file is a configuration file that is executed when a user logs in. The command echo 'export PATH=$PATH:/opt/operations1/bin' >> /etc
/profile will add the /opt/operations1/bin directory to the PATH variable for all users and allow them to access the application without using the full path. This is the correct command to use to resolve the issue. The other options are incorrect because they either overwrite the PATH variable (echo 'export PATH=/opt/operations1
/bin' >> /etc/profile) or do not use the correct syntax (echo 'export PATH=$PATH/opt/operations1/bin' >> /etc
/profile or echo 'export $PATH:/opt/operations1/bin' >> /etc/profile). References: CompTIA Linux+ (XK0-
005) Certification Study Guide, Chapter 9: Working with the Linux Shell, page 295.

The ip link show eth1 command can be used to check the link status of a network interface named eth1 in a Linux server. It will display information such as the MAC address, MTU, state, and flags of the interface. The ifconfig hw eth1 command is invalid, as hw is not a valid option for ifconfig. The netstat -r eth1 command would display the routing table for eth1, not the link status. The ss -ti eth1 command would display TCP information for sockets associated with eth1, not the link status. References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 13: Networking Fundamentals, page 436.

The oom-killer is invoked when the system runs out of memory, as indicated by the memory usage in the free
-m output. Since swap space is not being used and physical memory is almost entirely consumed, adding more memory resources will prevent the system from running out of memory and invoking the oom-killer to terminate processes.

To enable LUKS on a USB storage device with an ext4 filesystem format, the administrator needs to perform the following steps:
* Unmount the device if it is mounted using umount /dev/sdc (D)
* Create a partition table on the device using fdisk /dev/sdc (E)
* Format the partition with LUKS encryption using cryptsetup luksFormat /dev/sdc1 (H)
* Open the encrypted partition using cryptsetup luksOpen /dev/sdc1 LUKS0001
* Create an ext4 filesystem on the encrypted partition using mkfs.ext4 /dev/mapper/LUKS0001
* Mount the encrypted partition using mount /dev/mapper/LUKS0001 /mnt References:
* [CompTIA Linux+ Study Guide], Chapter 9: Securing Linux, Section: Encrypting Disks
* [How to Encrypt USB Drive on Ubuntu 18.04]

The administrator should add REQUIRES=mariadb.service to the HTTP server .service file to ensure that the database is available before the web application is started. This directive specifies that the HTTP server unit requires the MariaDB server unit to be started before it can run. If the MariaDB server unit fails to start or stops for any reason, the HTTP server unit will also fail or stop. This way, the dependency between the web application and the database is enforced by systemd.
The other options are not correct directives for accomplishing this task. TRIGGERS=mariadb.service is not a valid directive in systemd unit files. ONFAILURE=mariadb.service means that the HTTP server unit will start only if the MariaDB server unit fails, which is not what we want. WANTEDBY=mariadb.service means that the HTTP server unit will be started when the MariaDB server unit is enabled, but it does not imply a strong dependency or ordering relationship between them. References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 10: Managing Services with systemd; systemd.unit(5) - Linux manual page