The correct answer is option 4 - cirro-cumulus.

Mackerel sky is a type of cloud formation that is associated with cirro-cumulus clouds. Cirro-cumulus clouds are high-level clouds that appear as small, white, and patchy cloud formations with a wavy or honeycomb-like pattern. These clouds are usually found at altitudes above 20,000 feet and are composed of ice crystals due to their high altitude.

Option 1 - cumulo-nimbus clouds are tall and dense clouds that are associated with thunderstorms. They have a vertical development with a characteristic anvil-shaped top. Cumulo-nimbus clouds are not associated with mackerel sky formations.

Option 2 - strato-cumulus clouds are mid-level clouds that appear as a layer or sheet-like formation. These clouds are usually gray and can cover a large portion of the sky. Strato-cumulus clouds are also not associated with mackerel sky formations.

Option 3 - alto-cumulus clouds are mid-level clouds that appear as a layer or patchy formation with individual cloud elements. These clouds are usually white or gray and can have a mackerel-like appearance. However, mackerel sky is specifically associated with cir