Explanation:
The correct answer is option 'B' - MEM commands.

Memory in a computer system:
The total memory of a computer system refers to the amount of physical memory available in the system. It determines how much data and instructions can be stored and accessed by the computer.

MEM commands:
The MEM command in a computer system is used to display information about the memory configuration and usage. It provides details about the total memory, available memory, and memory usage by different components and processes in the system.

How MEM commands help in determining the total memory:
When you execute the MEM command, it displays information about the memory in the system, including the total memory size. The specific command may vary depending on the operating system being used. Some common examples of MEM commands are:

- In Windows: The MEM command is not available in Windows. However, you can use the systeminfo command to get information about the total physical memory. By opening the command prompt and typing "systeminfo" followed by Enter, it will display various system information including the total physical memory.

- In Linux: The free command can be used to display memory information. By typing "free -h" in the terminal, it will show the total memory size in a human-readable format.

- In macOS: The top command can be used to display memory information. By opening the terminal and typing "top" followed by Enter, it will display real-time information about system processes and memory usage. The total memory size is displayed at the top of the output.

Conclusion:
To determine the total memory of a system, the MEM command can be used in different operating systems. It provides information about the memory configuration and usage, including the total memory size.