Explanation:

The "Microprocessor" (also known as the CPU or Central Processing Unit) has the most direct connection to the motherboard, typically sitting in a dedicated socket (e.g., LGA or PGA) on the board itself. It is the brain of the computer and requires high-speed, direct communication with other motherboard components. A "Hard disk" (a) connects via an interface like SATA or IDE, which are ports on the motherboard, but the drive itself is a separate component. A "VDU" (Visual Display Unit or monitor) (b) connects via a video port (e.g., HDMI, DisplayPort), which is an output interface. A "Modem" (d) connects via a network port (Ethernet) or a USB port, or historically, an expansion slot. Therefore, the microprocessor has the most direct and integral connection to the motherboard.