The contractile vacuole of a paramecium should be active when the paramecium is in

a hypotonic environment

Explanation:

Contractile vacuoles are the membrane-enclosed structures present in certain freshwater protists such as Paramecium. The contractile vacuoles serve in osmoregulation in these organisms. The solute concentration of freshwater is much less than that of the aquatic organisms present in freshwater habitats. This means that the freshwater Paramecium is found in a hypotonic medium.

Water from the surroundings enters the body of Paramecium through the process of osmosis. The continuous inflow of water can cause the bursting of their cell. Contractile vacuoles prevent it by collecting the excess water and then contracting to expel the water continuously out of the cell through a pore. Therefore, a Paramecium would have an active contractile vacuole when present in a hypotonic medium.