The correct option is Nimbus.
Explanation
Clouds are classified according to their height, expanse, density, and transparency or opaqueness. The four primary types of clouds are Cirrus, Cumulus, Stratus, and Nimbus. Their altitude determines their classification into high, middle, and low clouds.
Detailed Analysis
- Cirrus (Incorrect): These are high-altitude clouds formed at 8,000 to 12,000 meters. They are thin, detached, and have a feathery appearance. They are always white and do not form near the ground.
- Altocumulus (Incorrect): These are middle-altitude clouds. They typically appear as layers or patches of rounded masses or rolls.
- 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. A defining characteristic is that they are sometimes so low that they seem to touch the ground. They are shapeless masses of thick vapour associated with precipitation.
- Cirrostratus (Incorrect): These are high-altitude clouds that appear as a thin, whitish veil covering the sky. They are known for producing a "halo" effect around the sun or moon but do not descend to the ground.
Key Takeaway
Nimbus clouds are distinct for their dark, shapeless appearance and their formation at very low altitudes, often appearing to touch the ground, unlike high-altitude clouds like Cirrus or Cirrostratus.