Which command will help in real-time monitoring of network traffic on a Junos OS device?

The command "monitor traffic" is used for real-time monitoring of network traffic on a Junos OS device. This command initiates a continuous display of traffic statistics, showing live data flows and bandwidth usage across the network interfaces. It provides valuable insights for network administrators, allowing them to assess performance, detect issues, or analyze trends as they happen.

In contrast, the other options do not serve the same purpose. For example, some commands, while possibly sounding similar, either do not exist in the Junos OS context or relate to different functions and utilities within the system, making them unsuitable for real-time monitoring of network traffic. Thus, "monitor traffic" stands out as the appropriate choice for this task.