Which command is used to view memory usage on a system?

The command that is used to view memory usage on a system is "free." When you run this command in the terminal, it provides a summary of the system's memory usage, including total memory, used memory, free memory, shared memory, buffer/cache memory, and available memory. This comprehensive overview helps users monitor how much memory is currently being utilized and how much is still available for use.

The other options serve different purposes: "ps" is primarily used to view the currently running processes on the system, offering information about process IDs, memory usage, and CPU usage associated with those processes. "route" is used to display or modify the routing table, which determines how network packets are directed through a network. "netstat" is used for networking statistics, allowing users to see active connections, routing tables, interface statistics, etc. Thus, these commands do not provide insights into memory usage directly, making "free" the correct choice for this specific inquiry.