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

The `date` command displays or sets the system date/time, but the arguments `$d $t $24` are not a valid or standard way to synchronize with an external time source.

`settime` is not a standard Linux command for synchronizing time with an NTP server, and the arguments are invalid for setting time from a remote source.

The `date` command does not accept input redirection (`<`) from an NTP server's hostname to synchronize time; a proper NTP client utility is required.

`/usr/sbin/runcron` is not a standard utility for running cron jobs or synchronizing time, and `date <ntp1.digex.net` is not a valid time synchronization method.

The `ntpdate` command is a command-line utility specifically designed for one-time synchronization of the system clock with an NTP server, making it suitable for a scheduled cron job. The `> /dev/null 2>&1` redirection is a standard practice in cron entries to suppress command output and prevent unnecessary email notifications.