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

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

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

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

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

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