Which command will configure a switch port to use the IEEE standard method of inserting VLAN membership information into Ethernet frames?

To configure a switch port to use the IEEE standard method of inserting VLAN membership information into Ethernet frames, you would use the dot1q encapsulation method. This method inserts VLAN tags into Ethernet frames according to the IEEE 802.1Q standard, allowing for the transmission of VLAN information across the network.

Options A, B, and D are incorrect because:
Option A: switchport trunk encapsulation isl configures the switch port to use the Inter-Switch Link (ISL) encapsulation method, which is a Cisco proprietary protocol and not part of the IEEE standard.
Option B: switchport trunk encapsulation ietf is not a valid command. There is no encapsulation method called "ietf" for trunk ports in Cisco switches.
Option D: switchport trunk encapsulation isl is a command used at the interface configuration level (Switch(config-if)#), but it specifies the ISL encapsulation method, which, as mentioned earlier, is not part of the IEEE standard.

Therefore, the correct command to configure a switch port to use the IEEE standard method of inserting VLAN membership information into Ethernet frames is Option C: Switch(config)# switchport trunk encapsulation dot1q.