Answer & Explanation
D) Switch (config-if) #switchport trunk encapsulation dot1q
If you are on a 2950 switch, then the interface command is just swichport mode trunk since the 2050 can only run the IEEE 802.1q version. However, a 3550 can run both ISL and 802.1q, so you must also use the encapsulation command. The argument to choose 802.1q for a trunking protocol is dot1q.