Which protocol is primarily used for monitoring and managing network devices?

The primary protocol used for monitoring and managing network devices is SNMP, or Simple Network Management Protocol. This protocol is specifically designed for network management, allowing administrators to monitor network performance, find and solve network problems, and plan for network growth. SNMP operates through a management information base (MIB), which is a database used for managing the elements in a network. Through SNMP, network devices like switches, routers, and servers can send notifications about their status and performance to a network management system, which significantly enhances the ability to oversee a network's health and efficiency.

The other options serve very specific functions that do not align with monitoring and managing network devices. SMTP, for example, is used for sending emails, while FTPS provides secure file transfer capabilities. NTP, or Network Time Protocol, is primarily focused on synchronizing the clocks of network devices, which is important but does not extend to monitoring or managing the devices themselves.