Option C, form and height, is the right answer.
A cloud is an aggregation or accumulation of little water drops and icicle crystals that are dissolved in the planet atmosphere. They are bodies that consist of enormous density and dimensions and therefore it is apparent to unprotected eyes. The clouds are classified on the basis of their shape and height. Cirrus, Cumulus and Stratus are the three types of Clouds. These clouds are further divided on the basis of their altitude or height.