Explanation
The character that, added to the end of a command, runs that command in the background as a child process of the current shell is the ampersand symbol (&). This means that the command will not block the shell and the user can enter other commands while the background command is running. The background command will also not receive any input from the keyboard or the terminal. The shell will print the job number and the process ID of the background command, and the user can use the jobs command to list the status of all background jobs in the current shell session. To bring a background job to the foreground, the user can use the fg command with the job number or the process ID. To terminate a background job, the user can use the kill command with the process ID.
The other characters are not valid or relevant for running a command in the background. The exclamation mark (!) is used to access the command history or to negate a condition. The plus sign (+) is used for arithmetic operations or to append text. The percent sign (%) is used for arithmetic operations or to refer to a job number. The hash sign (#) is used for comments or to specify a hexadecimal number.
References:
* [LPI Exam 101 Detailed Objectives], Topic 103: GNU and Unix Commands, Objective 103.5: Create, monitor and kill processes, Weight: 4, Key Knowledge Areas: Run jobs in the foreground and background.
* How to Run Linux Commands in Background, Topic: Using Ampersand.