Clouds are grouped on the basis of two criteria: and A. time required to form; composition B. color; size C. form; height D. temperature; reflectance

form and height

Explanation: Cloud consists of billions of little water droplets and/or ice crystals. Clouds are grouped on the basis of two criteria according to Luke Howard's system which are form and height. Form has three groups and height has three groups as well

In terms of form or shapes, Cirrus clouds are high, white, and thin. Cumulus clouds are globular, usually with a flat base while Stratus have several layers covering most of the sky. In terms of height, low clouds form nearest to the earth's surface up to about 2000meters and are generally made up of water droplets; middle clouds form between 2000-6000meters and may be made up of water droplets or ice crystals depending on the time of year; high clouds form above 6000meters and are generally made up of ice crystals with their temperature below freezing.