What does the expansion bus allow?

The expansion bus serves as a communication pathway that allows the CPU to interact with peripheral devices, such as printers, keyboards, mice, and external storage. It is a crucial component in the computer architecture that facilitates the exchange of data between the central processing unit and various peripherals connected to the system.

The expansion bus enhances the system's capabilities by enabling these devices to send and receive information as needed, which supports a range of functionalities and expands the overall usability of the computer. This capability is essential for interacting with external hardware to perform tasks that the CPU alone cannot fulfill.

In contrast, the other options represent different types of communication that do not primarily involve the expansion bus. For instance, communication between the CPU and RAM is typically handled by the memory bus, while data transfer within the CPU does not require a bus because it happens internally, involving the CPU's registers and internal pathways. Accessing the hard drive involves specific interfaces like SATA or IDE rather than the expansion bus itself, which specifically pertains to peripheral communication.