A long-running process is a program that takes a long time to complete or runs indefinitely on a Linux system.
A foreground process is a process that runs in the current terminal and receives input from the keyboard and output to the screen. A background process is a process that runs in the background and does not interact with the terminal. A background process can continue running even if the terminal is closed or disconnected.
To start a long-running process in the background, the user can append an ampersand (&) to the command, such as someapp &. This will run someapp in the background and return control to the terminal immediately.
To move a long-running process from the foreground to the background, the user can use two keystrokes:
Ctrl+Z and bg. The Ctrl+Z keystroke will suspend (pause) the foreground process and return control to the terminal. The bg keystroke will resume (continue) the suspended process in the background and detach it from the terminal. The statement B is correct.
The statements A, C, and D are incorrect because they do not perform the desired task. The bg keystroke alone will not work unless there is a suspended process to resume. The Ctrl+B keystroke will not suspend the foreground process, but rather move one character backward in some applications. The jobs keystroke will list all processes associated with the current terminal. The bg & keystroke will cause an error because bg does not take any arguments. References: [How to Run Linux Processes in Background]

To unlock an account that has been locked due to login failures, the administrator can use the command pam_tally2 --user=dev2 --reset (D). This will reset the failure counter for the user "dev2" and allow the user to log in again. The other commands will not unlock the account, but either display or increase the failure count. References:
* [CompTIA Linux+ Study Guide], Chapter 4: Managing Users and Groups, Section: Locking Accounts with pam_tally2
* [How to Lock and Unlock User Account in Linux]

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.

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.