Answer:
Answer 1: C. Stratus
Stratus is the type of cloud which covers whole sky. These clouds are thin and appear in form of white sheets. As they are very thin they cannot produce much rain and snow.
Answer 2: B. Nimbus
Nimbus are dark, gray clouds. They are thick and often blot the sunlight. They are responsible for causing rains.
Answer 3: C. At lower altitudes
Fogs are low lying clouds. Fog includes water vapor and air which is comparitively cooler than that found in clouds. Therefore, cooler air and water vapor forms a mass of fog. The fog formation is influenced by nearby water body, wind conditions and topography.