What is the epidermal cell modification in plants which prevents water loss?

What are the cells that make the leaves curl in plants during water stress?

During water stress, the bulliform cells
$(a)$ Become turgid
$(b)$ Become flaccid
$(c)$ Make the leaves curl inwards
$(d)$ Make the leaf surface exposed
The correct options are

Grass leaves curl Inwards during very dry weather. Select the most appropriate reason from the following

Bulliform or motor cells are present in