_______ are the stacks of membrane bound flattened sac-like structures.

Explanation

**Step-by-Step Solution:**

1. **Identify the Structure**: The question asks for a specific structure that is described as "stacks of membrane-bound flattened sac-like structures."

2. **Recall Relevant Biological Concepts**: In plant biology, we know that certain structures are involved in processes like photosynthesis. One of these structures is known for its role in the light-dependent reactions of photosynthesis.

3. **Define the Structure**: The structure that fits the description of being a stack of membrane-bound flattened sac-like structures is called "thylakoids."

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