Final answer:
If the red arrow is pointing to a stacked structure within the chloroplast, it is indicating a granum, which is a stack of thylakoid membranes.
Step-by-step explanation:
If the red arrow in the picture is pointing to a stack of disc-shaped structures within the chloroplast, then it is pointing to a granum (plural: grana). A granum is a stack of thylakoid membranes. The space surrounding the granum is called the stroma, which is where the light-independent reactions of photosynthesis take place. Thylakoids contain chlorophyll, which is critical for the light-dependent reactions of photosynthesis. Unlike the stroma, the thylakoid membranes are where the light harvesting reactions occur. It's important not to confuse the stroma with the stomata, which are openings on the leaf surface that allow for gas exchange; they are not directly related to the chloroplast structure.