To solve this problem, we need to identify the structures formed by stacking organized flattened membranous sacs in chloroplasts. Let's analyze each option:$\newline \bullet$Option 1: Cristae$\newline$ - Cristae are the folds of the inner mitochondrial membrane. They increase the surface area for chemical reactions in mitochondria.$\newline$ - Therefore, cristae are not related to chloroplasts.$\newline \bullet$Option 2: Grana$\newline$ - Grana are stacks of thylakoid membranes found within chloroplasts.$\newline$ - Thylakoids are the site of the light-dependent reactions of photosynthesis.$\newline$ - Hence, grana are the structures formed by stacking organized flattened membranous sacs in chloroplasts.$\newline \bullet$Option 3: Stroma lamellae$\newline$ - Stroma lamellae are extensions that connect grana stacks and are involved in the transport of materials.$\newline$ - They are not stacks of thylakoid membranes.$\newline \bullet$Option 4: Stroma$\newline$ - Stroma is the fluid-filled space surrounding the grana within the chloroplast.$\newline$ - It contains enzymes for the Calvin cycle but is not a stack of membranes.$\newline$Based on the analysis, the correct answer is Option 2: Grana.