Answer:
Breakfast is being served in the dining room.
Step-by-step explanation:
Passive voice has to show that the act (in this example, serve) is being performed upon the subject (breakfast). It has to include the correct tense of the verb to be and the past participle.
The tense of the original sentence is present continues (we learn that from the part where it says are serving), so we are changing the verb to be into the present continues and making it into is being.
The past participle of the verb serve is served, so we are changing serving from the original sentence to it.
In the end, we are adding additional information. In this case, it is the place where the action is performed: in the dining room.