Option A is wrong because nowhere 'fog' is referenced to any type of cloud in paragraph.
Option B is wrong because it is easy to predict whether from cloud but it is not referenced from given paragraph.
Option C is wrong because paragraph talks about the colour of all clouds but size of only 'stratus' cloud. So, whether size of all clouds are same or different, we can not conclude.
But one thing is sure that all clouds are different type because at least we can differentiate them based on their colours .
So, we are left with option(D). D is correct because the passage talks about $4$ cloud types.