First, when a port of a switch receives a data frame, it will first check its cam table to see if there is a corresponding destination mac address, and if there is, it will forward it. Otherwise, the switch will flood all ports except the receiving port and ask who owns this MAC address. Flooding will send a broadcast, and other connected switches will do the same after receiving the broadcast. Check their cam tables first, if not, continue flooding.

To sum up, I think D should be chosen for this question.

You can always ask me questions.