Two lines cross each other in a graph and use the same colour. What would best help the viewer?

Correct answer: Use distinct colours or line styles and direct labels

The governing principle is visual distinction: each data series should remain identifiable, even where lines overlap or cross. Different colours, dashed or solid patterns, markers and direct labels give the viewer more than one way to track each line. Making the graph smaller reduces readability, while removing the title or axes removes useful context. Thus distinct styles and direct labels are the best improvement.