Which of the following is a key component of a server?

The motherboard is a critical component of a server because it serves as the main printed circuit board (PCB) that connects all other hardware components. It allows the communication between the central processing unit (CPU), memory, storage devices, and other peripherals essential for server operation. Without a motherboard, the server cannot function as it provides the infrastructure needed for these components to work together effectively.

The other options do not serve as key components of a server. A smartphone is a mobile device focused on communication and personal use, not a server component. A router, while important for networking and connecting devices to the internet, is not a part of a server's internal architecture. Similarly, a desktop computer, although similar in some aspects to a server, is typically designed for individual use and does not share the same components or architecture aimed specifically at server functionalities and performance.