Which of the following components found in a legacy server has a 16-bit bus?

USB is a serial peripheral interface standard, not a motherboard expansion slot bus, and does not use a 16-bit parallel bus architecture.

PCI (Peripheral Component Interconnect) is a 32-bit expansion bus, which is wider than ISA.

ISA was originally an 8-bit bus that was extended to 16-bit in the AT form factor, making it the defining legacy 16-bit bus architecture. It was the dominant expansion standard through the 1980s and 1990s before being replaced by PCI. No other choice in this list operates at a 16-bit bus width.

AGP (Accelerated Graphics Port) is a 32-bit dedicated graphics slot and was never a 16-bit bus.