By default, which of the following is NOT a legitimate Linux run level?

By default, which of the following is NOT a legitimate Linux run level?

• 4: Normal mode
• 6: Reboot system
• 1: Single-user mode
• 3: Multi-user mode with networking 

EXPLANATION

A Linux run level defines the state of the system after boot. Conventionally, seven run levels exist, numbered from zero to six. They include:
0: Halt system
1: Single-user mode
2: Basic multi-user mode without NFS
3: Multi-user mode with networking
4: Unused
5: Multi-user mode with GUI

6: Reboot system

SOURCE