Answer:
The most appropiate method for announcing the promotions would be:
c. videoconferencing.
Explanation:
Videoconferencing will be the most appropiate method considering that they want to capture and gauge the emotional joy and delight on the faces of those promoted and from the options given the only one that will allow to see in real time the people promoted is this method as written documents, email and bulleting boards don't allow to see the person at the moment in which he/she will find out about the news.