The best answer is "a" - in the stroma.
In the chloroplast are two membranes, and inside the inner membrane is the gelatinous matrix known as the stroma. the stroma contains DNA and ribosomes. Membranous sacs thylakoids are found arranged in stacks called grana, therefore grana are simply stacks of thylakoids.
Within the chloroplast, grana resemble stacks of green discs or pancakes and are not necessarily uniform in their height or distribution.
The shape of thylakoids stacked in grana allows for optimum surface area.