Final answer:
The sacs called thylakoids are suspended in the stroma of chloroplasts and form stacks known as grana. The correct answer to the question is a) Grana; separates thylakoids.
Step-by-step explanation:
Suspended in the stroma of chloroplasts is a third membrane system comprising sacs called thylakoids. Each stack of thylakoids is known as a granum (plural = grana). The thylakoids are where light energy is harvested, and the stroma is the site where sugar is synthesized during the process of photosynthesis. The proper answer to the question is: a) Grana; separates thylakoids. Grana refers to the stacks of thylakoid membranes, and the thylakoids themselves are the membranous sacs that are vital for the light-dependent reactions of photosynthesis.