A Linux administrator needs to switch from text mode to GUI. Which of the following runlevels will start the GUI by default?

Runlevel 3 is typically defined as the multi-user text-mode environment with networking, without starting the graphical user interface.

Runlevel 4 is often undefined or can be custom-configured by the administrator, but it is not the default runlevel for starting the GUI.

In traditional SysV init systems, Runlevel 5 is conventionally defined as the multi-user graphical mode, which starts the X Window System and desktop environment, thus initiating the GUI by default.

Runlevel 6 is defined as the reboot runlevel, which brings the system down and restarts it, not for switching to or staying in a GUI environment.