Preferred server
A router can be configured to prefer an NTP source over another. A preferred server's responses are discarded only if they vary dramatically from the other time sources. Otherwise, the preferred server is used for synchronization without consideration of the other time sources. Preferred servers are usually specified when they are known to be extremely accurate. To specify a preferred server, use the prefer keyword appended to the ntp server command. The following example tells the router to prefer TimeServerOne over TimeServerTwo:
Router#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#ntp server TimeServerOne prefer
Router(config)#ntp server TimeServerTwo
Router(config)#^Z
Incorrect Answers:
A: In this case 10.1.1.1 will be used as the primary NTP server, and 10.1.1.2 is configured for redundancy and will be used if 10.1.1.1 is unreachable.
B: ntp server 10.1.1.2 primary is not a valid command
D: ntp server 10.1.1.1 fallback is not a valid command.
Reference: Hardening Cisco Routers By Thomas Akin February 2002 0-596-00166-5, Chapter 10, NTP.