Which NTP command configures the local device as an NTP reference clock source?

The `ntp peer` command configures the device to synchronize with a specified NTP peer, where both devices can synchronize to each other.

The `ntp broadcast` command configures the device to send or receive NTP time updates using broadcast messages, not to be an authoritative reference clock itself.

The `ntp master` command configures the local device to be an authoritative NTP server, allowing it to provide time services to other NTP clients even when it is not synchronized to an external reference clock itself.

The `ntp server` command configures the device to synchronize its clock with a specified upstream NTP server, not to act as an authoritative master reference clock when unsynchronized.