(A) : Clouds appear white due to scattering of light 
(R) : When size of objects are large compared to wavelength, all wavelengths scatter equally.

The clouds consist of dust particles and water droplets. Their size is very large as compared to the wavelength of the incident light from the sun. So there is very little scattering of light.Hence the light which we receive through the clouds has all the colours of light. As a result of this, we receive almost white light. Therefore, the cloud are generally white.