Cells lining the alveoli secrete a soapy substance known as:

Alveolar surfactant

Surfactant Alveolar is a type of substance secreted by pneumocytes II (one type of cell that forms the epithelium in the alveolar membrane). This substance  is known as an tensioactive agent, decreasing the surface tension within the alveoli.

Nimbostratus is the correct answer.
explanation: nimbostratus clouds are the type of clouds that are mostly likely present on gray, rainy day. these clouds produce continuous rain, snow and there is no thundering or lightening either.