In Amoeba and Paramecium, the cell organelle for osmoregulation is

The correct answer is contractile vacuole.

Osmoregulation is the process by which an organism regulates the concentration of water and solutes (such as salts) in its body to maintain osmotic balance. This is crucial for freshwater single-celled organisms like Amoeba and Paramecium.

Since the cytoplasm of Amoeba and Paramecium has a higher solute concentration than the surrounding freshwater environment, water constantly enters their cells by osmosis. To prevent the cells from swelling and bursting, they must actively expel excess water.

The specialized organelle responsible for this function is the contractile vacuole. It works through a cycle of accumulation and release:
1. Diastole: The contractile vacuole slowly collects excess water and some waste products from the cytoplasm, gradually expanding in size.
2. Systole: Once it reaches its maximum size, the vacuole moves to the cell membrane, fuses with it, and contracts to expel the accumulated water out of the cell.

Let's briefly review why the other options are incorrect:
- Nucleus: This organelle controls the cell's genetic material and regulates cellular activities, but it does not directly manage water balance.
- Body surface: While water enters the cell through the selectively permeable cell membrane (body surface) via osmosis, the membrane itself is not the organelle that actively pumps excess water out.
- Pseudopodia: These are temporary cytoplasm-filled projections used for locomotion and capturing food (phagocytosis) in Amoeba, not for water regulation.