What happens when the Linux kernel can’t mount the root filesystem when booting?

What happens when the Linux kernel can’t mount the root filesystem when booting?

A.
An error message is shown, stating that the corresponding kernel module couldn’t be
loaded.

B.
An error message is shown, showing which device couldn’t be mounted or informing that
init couldn’t be found.

C.
An error message is shown and the system reboots after a keypress.

D.
An error message is shown and the system boots in maintenance mode.

E.
An error message is shown and the administrator is asked to specify a valid root
filesystem to continue the boot process.