C. Sink until it reaches equilibrium and then remain at a constant depth.
Explanation:
An object immersed in a fluid experiences buoyant force. If the net buoyant force is greater than the net weigh of the object, the object will float.
If the net buoyant force is less than the net wight of the object, it will sink until a depth where the total weight of the object and the force due to the fluid above the object equals the net buoyant force at the bottom of the object (equilibrium), then the object remains at a constant depth.
