Which boot method requires that USB storage be bootable?

The correct answer is USB storage because this boot method involves using a USB device, like a flash drive or external hard drive, to initiate the booting process of a computer. For the system to recognize and successfully boot from the USB storage, the device must be configured to be bootable. This means it must contain the appropriate bootloader and operating system files in a format that the computer's BIOS or UEFI can recognize.

This contrasts with other methods. PXE, for example, involves network booting and does not rely on a local storage device, while the Internet-based booting requires a network connection to load the operating system from a remote server. Internal hard drives also do not require the same setup; they are typically already configured to boot the operating system installed on them. Hence, the necessity for USB storage to be bootable is specific to the USB method of booting.