Which of the following is the best definition of "granum?"
a) organelle in which photosynthesis takes place
b) fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of photosynthesis take place
c) stack of thylakoids located inside a chloroplast
d) disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of photosynthesis take place; stacks of thylakoids are called grana