The variable which represents the default time zone of the MySQL server is . . . . . . . .

In MySQL, the variable that represents the default time zone of the MySQL server is system_time_zone.
This variable holds the time zone setting configured for the MySQL server system.
Option A, time_zone, is incorrect because there is no such variable in MySQL representing the default time zone.
Option C, date_and_time, is incorrect because it does not represent the default time zone setting of the MySQL server.
Option D, system_time, is incorrect because it does not specifically represent the default time zone setting of the MySQL server; it might be related to the system time, but not the time zone configuration.