Scientists discovered a new species of freshwater fish that lives in a pond with a very low o2 level. when they measured the oxygen extraction efficiency of the fish, it was twice as high as that of closely-related fresh water fish. propose a hypothesis to explain how the fish achieve a high o2 extraction efficiency

In order to propose a hypothesis, there is a need to first see the function of gills in fishes. The gills of fishes comprise blood vessels that exhibit inherited tendencies of getting oxygen out of the water, which was consumed by fishes from their mouths. These gills also comprise thin walls, and when water moves over these walls of blood vessels, the oxygen from water moves into the blood, and then this oxygen-enriched blood goes to various organs.  

Thus, one of the hypotheses in the given case, can be the number of blood vessels, which are found in the gills of the mentioned freshwater fish to be higher in comparison to the blood vessels found in the normal fishes, and apart from this, the surface area of the thin walls, which are found in the gills is also more in the new species of freshwater fish.  

a physical change is being made.

the answer would be..

b. sucking