A new species of fish has just been discovered living in great numbers

Responding to a pm:

We need to look for an option which we can infer/conclude from the argument. The argument must provide data to support it. Since most of the options have come up for discussion, let's discuss each one of them.

A.To be placed into a certain classification, a fish must possess all the characteristics of that classification.

We cannot infer this from the argument. In fact, we can say that 'a fish needn't possess all the characteristics of that classification' with more authority. Note this line: "Because of the habitat where it was found and its obviously eel-like body, almost all scientists believe that the fish is an eel."
The scientists believe that the fish is an eel because of a couple of eel like characteristics. They are puzzled by one characteristic that this fish has which no eel has but other missing eel like characteristics doesn't seem to cause any problem at all. Hence we can certainly not conclude this statement.

B.Physical characteristics, such as skeletons, are scientists' primary means of classifying new species.

Again, we cannot infer this. We don't know the primary means of classifying new species. Scientists seem to be using physical characteristics e.g. eel like body, skeleton etc as well as habitat.

C.Some fish with cartilaginous skeletons are not sharks.

All we can say from the argument is that no eel has cartilaginous skeletons and some sharks have cartilaginous skeletons. We don't know whether some non-sharks have cartilaginous skeletons too. The new fish has not been classified yet. We don't know whether it is an eel or a shark or both. If it is classified as an eel, then we can say that some fish with cartilaginous skeletons are not sharks. If instead, it is classified as a shark, then we do not know whether there are some fish with cartilaginous skeletons that are not sharks. Hence, we cannot infer this.


D.The waters off Papua New Guinea are generally hospitable to eels.

We can infer this statement. The fish has been found in the waters off Papua New Guinea. The statement "Because of the habitat where it was found and its obviously eel-like body, almost all scientists believe that the fish is an eel." helps us infer that this habitat is suited to eels.

E.A fish cannot be both a shark and an eel.

The argument only says that the fish is either a shark or an eel. Recall that 'either or' construct implies 'at least one' in reasoning. Both are also possible. When we say 'I will get either a dog or a cat', it implies I will get at least one of the two and I could get both too.
In any case, the argument only talks about this particular fish. It says that this fish is either a shark or an eel. It is certainly possible that there is some fish which is considered both a shark and an eel. The argument doesn't say that such a fish does not exist. At the end of the debate, the scientists may put this fish also in both the classifications. We cannot conclude that a fish cannot be both from this argument.

Answer (D)