Option 1 -> Stratus clouds are low-level, uniform gray layers but not as thick and dark as described.
Option 2 -> Nimbus clouds are dark, thick, rain-bearing clouds that are extremely dense and block sunlight completely.
Option 3 -> Cirrus clouds are high, thin, wispy clouds made of ice crystals - white and translucent, not dark or dense.
Option 4 -> Cumulus clouds are puffy, white, cotton-like clouds that are typically not shapeless or opaque.
Hence, Nimbus clouds -> The term "nimbus" specifically refers to dark, rain-bearing clouds that form thick, shapeless masses. These clouds are so dense they completely block the sun's rays, appearing black or dark gray. They can form at various levels and are associated with precipitation -> correct