Cumulus and stratus clouds belong to which cloud group?

Cumulous and stratus belongs to low level clouds together with cumulonimbus and stratocumulus which are around 6500 feet from the ground.

=> Stratus refers to the clouds that are flats and spread out. They sometimes go down on grounds.
=> Cumulus is a kind of clouds that are life cauliflower in shape. They look like cottons in the sky.
=> Stratocumulus refers to clouds that are flat but a bit darker.
=>  Cumulonimbus are clouds that appears when rain is coming.

The name may be in importnce  to them

Ithink its b but i'm not sure.