What runlevels should never be declared as the default runlevel in /etc/inittab?

A.
1

B.
3

C.
5

D.
6

Explanation:
Standard Runlevel are:
0 – halt (Do NOT set initdefault to this)
1 – Single user mode
2 – Multiuser, without NFS (The same as 3, if you do not have networking) 3 – Full multiuser mode
4 – unused
5 – X11
6 – reboot (Do NOT set initdefault to this)
1 means single user mode and 6 means reboot. Which are not recommended for default runlevel.