Which of the following crontab entries could be used to set the system time at regular intervals?

The `date` command alone is for displaying or manually setting the time and does not inherently synchronize with an external time source like an NTP server.

`settime` is not a standard command-line utility used for synchronizing system time from an NTP server.

The syntax `date<ntp1.digex.net` is incorrect for synchronizing time; the `date` command does not accept an NTP server hostname via standard input.

`/usr/sbin/runcron` is not a standard utility for time synchronization, and the command `date <ntp1.digex.net` is still syntactically incorrect for synchronizing with an NTP server.

The `ntpdate` command synchronizes the system's clock with a specified NTP server, making it suitable for a cron job that needs to regularly update the system time. The redirection `> /dev/null 2>&1` suppresses all command output, which is standard practice for cron entries.