The correct option is Nimbus.
Explanation
Clouds are classified according to their height, expanse, density, and transparency or opaqueness. There are four primary types of clouds: Cirrus, Cumulus, Stratus, and Nimbus. Each type has distinct physical characteristics and formation altitudes.
Detailed Analysis:
- Nimbus (Correct): Nimbus clouds are black or dark gray. They form at middle levels or very near to the surface of the earth. They are extremely dense and opaque to the rays of the sun. In standard physical geography definitions, they are specifically described as "shapeless masses of thick vapour." The term "Nimbus" is associated with rain or precipitation.
- Cirrus: These form at high altitudes (8,000 - 12,000 m). They are thin and detached clouds having a feathery appearance. They are always white.
- Altostratus: These are mid-level clouds that appear as a fibrous veil or sheet, often grey or blue-grey, covering the sky.
- Stratocumulus: These are low-level clouds that appear as large, dark, rounded masses, usually in groups, lines, or waves.
Key Takeaway:
Nimbus clouds are distinguished by their dark, opaque, and "shapeless" structure, indicating a high concentration of water vapour and imminent precipitation.