Final answer:
To protect a Linux system from being booted into single-user mode, you should configure and use the Grub password security feature.
Step-by-step explanation:
If you wanted to protect a Linux system from being booted into single-user mode, you would configure and use the Grub password security feature. This involves setting a password for accessing the GRUB bootloader, which is responsible for starting the Linux operating system. By setting a strong password, you can prevent unauthorized access to single-user mode, which could be used to bypass certain security measures.
The other options mentioned are not directly related to protecting against single-user mode.