Which is the internal membrane of the chloroplasts that is organized into flattened membranous sacs ?

THYLAKOIDS the term used to describe the cholorophlasts internal membrane where it is organized into flattened membranous sacs.

The thylakoids are arranged in orderly stacks called grana that contain energy-transducing machinery.

The space inside the thylakoid sac is called lumen and the space outside the thylakoid sac but still within the chloroplast is called stroma.