The correct answer is option 3 - Stratus, Nimbus and Cumulus.

Low clouds are clouds that form at a relatively low altitude in the sky. These clouds are generally below 6,500 feet (2,000 meters) in altitude.

Option 1 - Altostratus, Altocumulus, and Nimbostratus are classified as middle-level clouds and are found at altitudes between 6,500 feet and 20,000 feet. They are not considered low clouds.

Option 2 - Cirrus, Cirrostratus, and Cirrocumulus are classified as high-level clouds and are found at altitudes above 20,000 feet. They are not considered low clouds either.

Option 4 - Cirrus, Cirrostratus, and Altostratus include both high-level and middle-level clouds, but not low clouds.

Therefore, option 3 is the correct answer as Stratus, Nimbus, and Cumulus are all examples of low clouds. Stratus clouds are flat and featureless and often cover the entire sky. Nimbus clouds are associated with precipitation, while Cumulus clouds are white and puffy with a flat bottom. They are often seen on sunny days.