To solve this question, the user needs to know basic commands to check system status. The user must choose the command that can show a one-line summary of system status.

Now, let's go through each option and explain why it is right or wrong:

A. uptime: This option is correct. The uptime command displays how long the system has been running, the number of users currently logged on, and the system load averages for the past 1, 5, and 15 minutes. The output is a one-line summary of system status.

B. pmon: This option is incorrect. There is no command named pmon in Linux or Unix systems.

C. top: This option is incorrect. The top command displays the system's processes in real-time and provides a dynamic, real-time view of the running system. It does not provide a one-line summary of system status.

D. df: This option is incorrect. The df command is used to display the amount of disk space available on file systems. It does not provide a one-line summary of system status.

The Answer is: A