正解:C
The trunk configuration issue between Switch1 and Switch2 over the back-to-back link can be resolved by ensuring both switches are configured to support trunking on their respective interfaces. Option C is the correct answer because it configures both interfaces as trunk ports with the command switchport mode trunk.
This command explicitly sets the switch port mode to trunk, overriding any dynamic negotiation, and ensures that the link between the switches will carry traffic for multiple VLANs.