It could be c or d because when you are chatting to someone through say Skype you use fonts and colors to identify who you are talking to. But in most cases it would be d
Explanation:
The statement which says that a graph is complete means an undirected graph whose every pair of vertices is connected by an particular edge or unique edge.
An undirected graph is a graph whose edges soea not contain any direction means to which vertex they are directed.
A complete graph has an edge between every pair of vertices that exist in that graph.
The maximum number of colors that should be used on a slide is 4.