Which bus connects the L2 cache to the processor?

The correct choice identifies the bus specifically designed to connect the Level 2 (L2) cache directly to the processor, which is the backside bus. The backside bus allows for a high-speed link between the L2 cache and the CPU, enabling quick data transfer without competing with the main system's data traffic. This is essential for improving the performance of the processor, as it can access cached data without delays associated with the main memory.

In contrast, the other options refer to different types of buses facilitating various forms of communication. The frontside bus generally connects the CPU to the main memory and other components on the motherboard, but it does not directly connect the L2 cache. The PCI and system I/O buses are primarily used for peripheral component interconnect and input/output operations, which do not pertain to the connection between the L2 cache and the processor. Each of these alternatives serves a distinct purpose, underscoring the unique role of the backside bus in enhancing CPU performance by allowing efficient communication with the cache.