The free -h command can be used to check the amount of free and used memory in the system in a human-readable format. This can help to troubleshoot the issue of mysqld being killed due to out of memory. The vmstat -a 1 4 command can be used to monitor the system's virtual memory statistics, such as swap usage, paging activity, and memory faults, every one second for four times. This can help to identify any memory pressure or performance issues that may cause out of memory errors. The nc -v 127.0.0.1 3306 command would attempt to connect to the MySQL server on port 3306 and display any diagnostic messages, but this would not help to troubleshoot the memory issue. The renice -15 $( pidof mysql ) command would change the priority of the mysql process to -15, but this would not prevent it from being killed due to out of memory. The lsblk command would display information about block devices, not memory usage. The killall -15 command would send a SIGTERM signal to all processes with a matching name, but this would not help to troubleshoot the memory issue. Reference: [CompTIA Linux+ (XK0-005) Certification Study Guide], Chapter 15: Managing Memory and Process Execution, pages 468-469.

Explanation
The following commands can help the user to authenticate and show the ticket grants using Kerberos SSO on a Linux server:
kinit: This command obtains and caches an initial ticket-granting ticket (TGT) for the user from the Kerberos key distribution center (KDC). The user needs to enter their password or use a keytab file to authenticate1.
klist: This command lists the cached tickets, including the TGT and any service tickets, for the user. It also shows the expiration time and flags for each ticket2.
For example, the user can run the following commands to log in and view their tickets:
$ kinit username@REALM
Password for username@REALM:
$ klist
Ticket cache: FILE:/tmp/krb5cc_1000
Default principal: username@REALM
Valid starting Expires Service principal
04/06/2023 16:06:59 04/07/2023 02:06:59 krbtgt/REALM@REALM
renew until 04/13/2023 16:06:59
References:
kinit(1) - Linux man page, section "Description".
klist(1) - Linux man page, section "Description".

Explanation
The command fsck.ext4 /dev/sda1 can be used to address the issue. The issue is caused by a corrupted filesystem on the /dev/sda1 partition. The error message shows that the filesystem type is ext4 and the superblock is invalid. The command fsck.ext4 is a tool for checking and repairing ext4 filesystems. The command will scan the partition for errors and attempt to fix them. This command can resolve the issue and allow the system to start. The other options are incorrect because they either do not fix the filesystem (partprobe or fdisk) or destroy the data on the partition (mkfs.ext4). References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 10: Managing Storage, page 325.

The command semanage port -a -t http_port_t -p tcp 6379 adds a new port definition to the SELinux policy and assigns the type http_port_t to the port 6379/tcp. This allows the web application to run on this port and accept connections from users. This is the correct way to resolve the issue. The other options are incorrect because they either delete a port definition (-d), use the wrong protocol (top instead of tcp), or list the existing port definitions (-l). References: CompTIA Linux+ (XK0-005) Certification Study Guide, Chapter 18:
Securing Linux Systems, page 535.

The snap list command lists all installed snap packages on the system. It provides details such as the package name, version, and installation status. This command is useful for verifying whether a specific package was installed using the snap package manager.