Which NTP mode must be enabled when a Cisco router is used as an authoritative NTP server?

When a Cisco router acts as an authoritative NTP server, it must be configured in server mode. This allows the router to provide time synchronization services to other NTP clients on the network. The server command, often used with the ntp master command, designates the router as a stratum 10 (or lower if specified) NTP server, making it a reliable time source. primary is not an NTP mode; it's a concept related to the ntp master command which declares the router as an authoritative time source. peer mode is used when two NTP devices exchange time information and synchronize with each other, but it doesn't inherently make one authoritative for others. broadcast client mode is for devices receiving broadcast NTP messages, not for serving time.