Q. Which of the following clouds are extremely dense and opaque to the rays of the sun?

[A] Nimbus clouds

[B] Cirrus clouds

[C] Cumulus clouds

[D] Stratus clouds

Answer: A
Notes:

Nimbus clouds are black or dark gray. They form at middle levels or very near to the surface of the earth. These are extremely dense and opaque to the rays of the sun.  

Sometimes, the clouds are so low that they seem to touch the ground. Nimbus clouds are shapeless masses of thick vapour. 

Source: NCERT XI Fundamentals of Physical geography