In Paramoecium, osmoregulation is a function of

The correct option is contractile vacuole.

Here is a step-by-step explanation of why this option is correct:

1. Understanding Osmoregulation: Osmoregulation is the process by which an organism regulates the concentration of water and solutes (like salts) in its body to maintain osmotic balance and prevent its cells from bursting or shrinking.

2. Habitat of Paramecium: Paramecium is a freshwater unicellular eukaryotic organism. Since its surrounding environment is hypotonic (contains a lower solute concentration than the inside of the cell), water constantly enters the cell via osmosis.

3. Role of the Contractile Vacuole: To prevent the cell from swelling and bursting due to the constant inflow of water, Paramecium uses specialized organelles called contractile vacuoles. These vacuoles actively collect excess water from the cytoplasm through radial canals, swell in size, and then contract to pump the water out of the cell through a temporary pore. This active regulation of water content is the primary mechanism of osmoregulation in the organism.

4. Analyzing the Other Options:
- Cytostome: This is the cell mouth used for ingestion of food particles.
- Cytopyge: Also known as the anal pore, it is used for the egestion of undigested waste.
- Trichocysts: These are rod-like structures embedded in the pellicle that can be discharged for defense or capturing prey, not for water regulation.

Thus, the contractile vacuole is the specific organelle responsible for performing osmoregulation in Paramecium.