Which of the following is used on the client to boot a Linux system from a TFTP server?

HTTP is a protocol for transferring hypertext documents, and while it can be used for downloading files, it's not the primary protocol for network booting a client system directly.

GRUB (Grand Unified Bootloader) is a bootloader program that loads an operating system, but it's loaded by PXE in a network boot scenario, not the mechanism that initiates the network boot itself.

EFI (Extensible Firmware Interface) or UEFI is a firmware interface that replaces BIOS, but it is the environment within which PXE may operate, not the network boot protocol itself.

PXE (Preboot eXecution Environment) is a client-server interface that allows a computer to boot directly from a network, independent of installed operating systems or data storage devices. When booting via PXE, the client typically fetches its initial bootloader files (like GRUB or iPXE) from a TFTP server.