CORRECT TEXT
Configuration of X is complete and now the default runlevel can be changed from runlevel 3 to
runlevel 5. In which of the following configuration files should this be done?

Answer: /etc/inittab

Explanation:

from the man pages:
inittab – init daemon configuration
The /etc/inittab file was the configuration file used by the original System V init(8) daemon.
The Upstart init(8) daemon does not use this file, and instead reads its configuration from files in
/etc/init. See init(5) for more details.