What is the method of applying watercolour on wet paper called?

Correct answer: Wet-on-wet

The governing concept is the condition of the surface and the paint. When wet watercolour is applied to paper that is already wet, the method is called wet-on-wet, so option A is correct. Moisture allows the pigment to spread softly and creates diffused edges. Wet-on-dry uses wet paint on dry paper and usually gives sharper edges; the other combinations do not describe this process.