Which command synchronizes time with an NTP server?

The command that synchronizes time with an NTP server is often the one that specifically references the procedure used to communicate with the NTP service. In this case, the command ntpdate ntp.server.com is correct because it directly invokes the ntpdate utility, which is commonly used in Unix-like systems to set the local system clock to the time from a specified NTP server.

When this command is executed, it queries the NTP server identified (in this case, ntp.server.com) to obtain the current time, and then it adjusts the system clock to match that time. The ntpdate command is particularly useful for initial setup or for systems that cannot run a continuous NTP daemon.

Other provided options do not correspond to standard commands for synchronizing time with an NTP server. Options like ntpupdate, ntp-sync, and clock sync are not recognized commands in standard Unix-like operating systems for time synchronization. The correct option is thus clearly identified as it uses the appropriate syntax and utility for the function of synchronizing time.