You need an additional email address for a user in your department. You decide to add just an alias on your sendmail email server. What command must be executed to make the changes take effect?

Answer: newaliases

Explanation:
From the man pages:
newaliases Initialize the alias database. If no input file is specified, the program processes the file(s) specified with the alias_database configuration parameter. If no alias database type is specified, the program uses the type specified with the default_database_type configuration parameter. This mode of operation is implemented by running the postalias(1) command.