In a chloroplast, little stacks of membranous sacs are called _____. Each individual sac is called a ______.

The correct option is A granum, thylakoid
The space limited by the inner membrane of the chloroplast is called the stroma.
A number of organized flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum).
In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen