Final answer:
A stack of thylakoids, which are disc-shaped structures within a chloroplast vital for photosynthesis, is known as a granum.
Step-by-step explanation:
A stack of thylakoids is called a granum (plural = grana). Thylakoids are disc-shaped, fluid-filled, membrane-bound sacs within a chloroplast. These structures are where the light-dependent reactions of photosynthesis occur. The interconnected thylakoids stack tightly to form grana, which are essential for the chloroplast's function in photosynthesis.