Which of these allows a server to establish an EtherChannel across two physical Cisco Catalyst 6500 Series switches?

Cisco StackWise is a physical stacking technology that uses a proprietary ring cable and is designed for smaller Catalyst platforms such as the 3750 and 9300 series - it is not supported on the Catalyst 6500.

VDC (Virtual Device Context) is a Cisco Nexus-specific feature for creating isolated logical switch instances within one chassis and is not available on the Catalyst 6500 platform.

vPC (virtual Port Channel) achieves multi-chassis EtherChannel on Cisco Nexus switches but is not a Catalyst 6500 feature and therefore cannot be deployed in this scenario.

VSS (Virtual Switching System) is a Cisco Catalyst 6500 and 6800 feature that joins two physical chassis into one logical switch using a Virtual Switch Link (VSL) for control and data synchronization. From the server's perspective, both chassis present a single switch identity, so a standard LACP or PAgP EtherChannel can bundle links that physically terminate on different chassis without requiring any special server-side configuration. This provides both bandwidth aggregation and chassis-level redundancy in a single EtherChannel bundle.