The command that would allow a user to choose a stronger password and set it on the existing SSH key file is ssh-keygen -p -f <keyfile>. This command uses the ssh-keygen tool, which is used to generate, manage, and convert authentication keys for SSH. The -p option stands for passphrase, and it allows the user to change or remove the passphrase of an existing private key file. The -f option specifies the filename of the key file. The command will prompt the user for the old passphrase, and then for the new passphrase twice.

Explanation
The slowness is caused by the CPU being overloaded. The iostat command shows that the CPU utilization is
100%, which means that there are more processes competing for CPU time than the CPU can handle. The other options are incorrect because:
The system is not running out of swap space, as shown by the iostat command, which shows that there is no swap activity (si and so columns are zero).
The memory is not exhausted, as shown by the free -m command, which shows that there is still available memory (avail column) and free buffer/cache memory (buff/cache column).
The processes are not paging, as shown by the vmstat command, which shows that there are no major page faults (majflt column) and no swap activity (si and so columns). References: CompTIA Linux+ Study Guide, Fourth Edition, page 417-419, 424-425.

In "/etc/sysconfig/network" You set the hostname:
HOSTNAME="myserver.local"
In "/etc/hosts" You bind an IP to resolve the name:
XXX.XXX.XXX.XXX myserver.com

Listing Rules as Tables
To output all of the active iptables rules in a table, run the iptables command with the -L option:
sudo iptables -L
Flushing Chains
For example, to delete all of the rules in the INPUT chain, run this command:
sudo iptables -F INPUT
https://www.digitalocean.com/community/tutorials/how-to-list-and-delete-iptables-firewall-rules

The permanent rule --add-service=ntp --permanent is added, but such changes only take effect after a firewall reload.
* D is correct: firewall-cmd --reload applies the permanent rule allowing traffic on UDP port 123, and systemctl restart ntpd ensures the NTP daemon is refreshed.
Incorrect Options:
* A, B: These commands do not apply firewall rules.
* C: --complete-reload is valid but overkill for applying one rule, and rarely used unless debugging.
Reference:
CompTIA Linux+ XK0-005 Study Guide, Chapter 5
man firewall-cmd
man ntpd