The maximum niceness value that a regular user can assign to a process with the nice command when executing a new process is 19. The niceness value is a user-space value that controls the priority of a process. The lower the niceness value, the higher the priority, and vice versa. The niceness value range is -20 to +19, where -20 is the highest priority and +19 is the lowest priority. The default niceness value is 0. The nice command can be used to run a new process with a modified niceness value. The syntax is: nice -n value command, where value is the niceness value and command is the process to run. For example, nice -n 10 sleep 60 will run the sleep command with a niceness value of 10 for 60 seconds. However, regular users can only increase the niceness value of their processes, not decrease it. This means that they can only lower the priority of their processes, not raise it. The minimum niceness value that a regular user can assign is 0, and the maximum is 19. Only the root user can assign a negative niceness value, which means raising the priority of a process. For example, nice -n -10 sleep 60 will run the sleep command with a niceness value of -10 for 60 seconds, but only if the user is root. The other options are not correct because:

A. 9 is not the maximum niceness value that a regular user can assign, but a valid niceness value within the range.

C. 49 is not a valid niceness value, as it exceeds the maximum of 19.

D. 99 is not a valid niceness value, as it exceeds the maximum of 19.