The suggested answer is B.
To display executing processes in real time, the 'top' command is the appropriate choice. 'top' provides a dynamic, real-time view of system processes, including their CPU and memory usage. The 'ps' command, on the other hand, generates a static snapshot of processes at the moment of execution, and does not update in real-time. 'nice' is used to set the priority of a process, not to display them. 'pstree' shows a tree of processes, but it is also a static representation. Therefore, to monitor processes as they happen, 'top' is the most suitable command.