The answer in your test has as one of its correct choices that VoIP uses a lot of bandwith. A CCENT book recommended by the site indicates that VoIP bandwith utilisation is low. Is that a misprint or is the question scoring incorrectly and is the real test scored the same way?
When you put things out of context, you will definitely be confused. The question asks for two services/applications that can cause slow down on a typical network. Chat, IM, web browsing etc. do not cause slow down because they do not demand constant connection to the other end. On the other hand, PC-based VoIP services and online video sites (streaming video) do use most of the bandwidth on a network.
So the comparison of bandwidth usage is between web browsing+chat applications and VoIP+streaming video applications.
So its the constant connection that casuses the bandwidth to be sucked up and that causes the slow down?
BWS