The (average) human body floats in water. SCUBA divers wear weights and a flotation vest that can fill with a varying amount of air to establish neutral buoyancy. Assume that a diver originally establishes neutral buoyancy at one depth. To establish neutral buoyancy at a lower depth, the diver should(A) let some air out of the vest.(B) add some air to the vest.(C) do nothing, because neutral buoyancy already exists.

The correct option is (A).
When an object floats in water, there is a buoyancy force which acts upwards on the objects. According to Archimedes’ principle, the magnitude of the buoyant force is depending on the density of the water and the volume of the object. Thus, the object will sink if we decrease the volume of the object to maintain the buoyancy. So, SCUBA divers can float in the water by varying the amount of air in the floatation vest. This changes their overall volume in the water. Now, the diver can establish neutral buoyancy at one depth by varying the volume of the air in the vest. If he wants to established neutral buoyancy at a lower depth, the volume of the vest should be change. This is possible by reducing the amount of air out of the vest. So, the overall volume decreases and the diver can go further depth. Therefore, option (A) is correct.
Other options are ruled out.