Explanation:

- Aquatic plants have specialized tissues for buoyancy.

- Large air sacs in these plants are crucial for floating.

- Parenchyma:

- Typically found surrounding these air sacs.

- Contains large intercellular spaces that aid buoyancy.

- Flexible and supports the plant's metabolic activities.

- Parenchyma is the correct choice.

- Collenchyma:

- Provides support to growing parts of plants.

- Not typically associated with air sacs in aquatic plants.

- Sclerenchyma:

- Offers rigid support.

- Lack of intercellular spaces, not suitable for buoyancy.

- Complex Tissue:

- Involves xylem and phloem.

- Does not directly relate to air sac's buoyancy function.