Explanation:

The correct option is (B) System bus. A system bus is a set of electrical pathways or conductors on the motherboard that facilitates communication between the Central Processing Unit (CPU) and other essential components. These components include Random Access Memory (RAM), graphics cards, storage controllers, and various input/output (I/O) devices. The bus acts as a shared communication channel, allowing data, addresses, and control signals to be transmitted efficiently across the system. Option (A) Input unit is a general term for input devices, not a specific component on the motherboard connecting the CPU. Option (C) ALU (Arithmetic Logic Unit) is a sub-component *within* the CPU, responsible for calculations, not for connecting the CPU to other components. Option (D) Primary memory (RAM) resides *on* the motherboard and communicates with the CPU *via* the system bus, but it is not the connector itself.