Which of the following can be used to optimize the bandwidth between connected switches?

Port groups typically refer to logical groupings of virtual machine network adapters on a virtual switch, not a mechanism to optimize bandwidth between physical switches.

Trunking, also known as link aggregation or EtherChannel, bundles multiple physical Ethernet links between two switches into a single logical link. This aggregation optimizes bandwidth by distributing traffic across all links in the bundle and provides fault tolerance if one link fails.

ISL (Inter-Switch Link) is a Cisco-proprietary trunking protocol for carrying multiple VLANs over a single link, but 'trunking' is the broader concept for bandwidth optimization itself.

Routing determines the path for network traffic between different networks and does not directly optimize bandwidth between two connected switches on the same network segment.