Which method is useful for making distant mountains appear light in watercolour?

Correct answer: Use a light, diluted colour

The governing concept is atmospheric perspective: distant forms are usually represented with lighter, less intense, and often cooler-looking colour than nearby forms. Diluting the watercolour with clean water lowers pigment concentration and creates a pale wash, making the mountains recede visually. A dark colour or thick black outline increases contrast and brings them forward, while oil is unsuitable for this medium. Thus, option C is correct.