The correct option is They look like cotton wool and have a flat base..
Explanation
Clouds are classified according to their height, expanse, density, and transparency or opaqueness. There are four primary groupings of clouds based on their appearance: Cirrus, Cumulus, Stratus, and Nimbus.
Option Analysis
- They have a feathery appearance and are always white. is incorrect: This description corresponds to Cirrus clouds. Formed at high altitudes (8,000 - 12,000 m), they are composed of ice crystals, possess a feathery appearance, and are always white in colour.
- They look like cotton wool and have a flat base. is correct: This description corresponds to Cumulus clouds. They look like cotton wool and generally have a flat base. They exist in scattered patches and are typically formed at a height of 4,000 - 7,000 m.
- They are layered clouds covering large portions of the sky. is incorrect: This description corresponds to Stratus clouds. These are layered clouds that cover large portions of the sky. They are often formed due to the loss of heat or the mixing of air masses with different temperatures.
- They are shapeless masses of thick vapour found very near the surface. is incorrect: This description corresponds to Nimbus clouds. They are black or dark gray and form at middle levels or very near the surface. They are characterized as shapeless masses of thick vapour.
Key Takeaway
Cumulus clouds are distinct for their vertical development, "cotton wool" texture, and flat bases, distinguishing them from the feathery Cirrus, layered Stratus, and dark, shapeless Nimbus clouds.