b. a recess typically made of irregular stones and covered with artificial foliage
Explanation:
A grotto is an artificial recess or structure made of irregular stones and covered with artificial foliage that make it look like a natural cave. It is mostly found in parks, outdoor gardens or near waters and is sometimes used to place statues of saints where people can go to pray to. In antiquity, for example, a grotto was seen as a dwelling place of divinities.
This is only an idea, but an advantage could be that you don't have to wait for it to dry and disadvantages could be that you don't get as vivid colours?
