A school direction sign uses only colours to indicate routes to classrooms. What is the best improvement for students with colour-vision difficulty?

Correct answer: Add symbols and written labels with colours

The governing concept is accessible visual communication, especially the use of redundant cues. Colour alone may not distinguish routes for students with colour-vision difficulty, even if the colours are brighter. Adding distinct symbols and written labels provides information through shape and language as well as colour, making the sign easier to interpret. Reducing the size worsens readability, while one colour removes route differentiation; therefore option C is correct.