The correct option is Cumulus.
Explanation
Clouds are classified based on their appearance, altitude, density, and transparency. The four primary types are Cirrus, Cumulus, Stratus, and Nimbus.
- Cumulus Clouds (Correct): These clouds resemble cotton wool. They typically form at heights between 4,000 and 7,000 meters. They exist in patches and are seen scattered across the sky. A distinguishing feature is their flat base.
- Stratus Clouds: As the name implies, these are layered clouds that cover large portions of the sky. They are generally formed due to the loss of heat or the mixing of air masses with different temperatures.
- Cirrus Clouds: These form at high altitudes (8,000 - 12,000 meters). They are thin, detached clouds with a feathery appearance and are always white in colour.
- Nimbus Clouds: These are black or dark gray clouds that form at middle levels or very near the surface. They are extremely dense and opaque to sunlight. They are shapeless masses of thick vapour.
Key Takeaway: Cumulus clouds are identified by their scattered, cotton-wool appearance and characteristic flat bases.