What should be done to make a darker red in watercolour?

Correct answer: Use more red pigment and less water

In transparent watercolour, colour depth depends on pigment concentration. Using more red pigment and less water creates a stronger, darker red wash, so option C is correct. More water spreads the pigment and makes the colour lighter. White is not normally used to lighten transparent watercolour because the paper provides the light; yellow would alter the hue toward orange instead of simply deepening red.