The correct crontab entry for executing myscript at 30 minutes past every hour on Sundays is D. 30 0-23 * * 0 myscript. This is because the crontab format consists of six fields: minute, hour, day of month, month, day of week, and command. The values for each field can be:
* A single number, such as 5 or 10.
* A range of numbers, such as 1-5 or 10-15.
* A list of numbers separated by commas, such as 1,3,5 or 10,12,14.
* An asterisk (*), which means all possible values for that field.
* A step value, which means every nth value for that field, such as */5 or 10-20/2.
The day of week field can be either a number from 0 to 6, where 0 and 7 are Sunday, or a three-letter abbreviation, such as SUN or MON. The month field can be either a number from 1 to 12, or a three-letter abbreviation, such as JAN or FEB.
In this case, the crontab entry D. 30 0-23 * * 0 myscript means:
* 30: Execute the command at the 30th minute of every hour.
* 0-23: Execute the command for every hour from 0 (midnight) to 23 (11 PM).
* *: Execute the command for every day of the month, regardless of the month.
* *: Execute the command for every month, regardless of the year.
* 0: Execute the command only on Sundays.
The other options are either incorrect or do not match the requirement. For example, option A. 0 * * * 30 myscript means:
* 0: Execute the command at the 0th minute of every hour.
* *: Execute the command for every hour of the day.
* *: Execute the command for every day of the month, regardless of the month.
* *: Execute the command for every month, regardless of the year.
* 30: Execute the command only on the 30th day of the week, which is invalid.
References:
* Crontab Explained in Linux [With Examples]
* 'crontab' in Linux with Examples - GeeksforGeeks
* Crontab Syntax on Linux + Useful Examples - Hostinger

server

/usr/sbin/sendmail, /usr/lib/sendmail

lpq, /usr/bin/lpq, lpstat, /usr/bin/lpstat