Explanation/Reference: Explanation: config)# ntp authentication-key 42 md5 aNiceKey Defines the authentication keys. The device does not synchronize to a time source unless the source has one of these authentication keys and the key number is specified by the ntp trusted-key number command. The range for authentication keys is from 1 to 65535. Cisco NX-OS Release 5.2(3) and later 5.x releases support up to 15 alphanumeric characters for the MD5 string. Earlier releases support up to 8 alphanumeric characters. (config)# ntp trusted-key 42 Specifies one or more keys that a time source must provide in its NTP packets in order for the device to synchronize to it. The range for trusted keys is from 1 to 65535. This command provides protection against accidentally synchronizing the device to a time source that is not trusted. B is not Correct because to enable NTP Authentication is required this command ntp authenticate. So A is Correct. Reference: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/system_management/ configuration/guide/sm_nx_os_cg/sm_3ntp.html#93976