Split the argument into premises and conclusion.

Premises:

In modern deep-diving marine mammals, such as whales, the outer shell of the bones is porous.
It helps them swim back to the surface after a deep dive.
The outer shell of the bones was also porous in the ichthyosaur.

Conclusion: ichthyosaurs were deep divers.

Based on the porous bones, we are concluding that Ichthy were deep divers. So we are saying that porous bones implies deep divers. We are saying that all porous bone marine animals were deep divers. We are not saying that all deep divers must have porous bones. We are saying that all animals with porous bones must be deep sea divers - that porous bones are sufficient to lead to deep sea diving. Because we are given porous bones, we are concluding deep sea diving.

We need to weaken this conclusion.

(A) Some deep-diving marine species must surface after dives but do not have bones with porous outer shells.

Doesn't impact our conclusion. We are not saying that all deep sea divers must have porous bones.
We are saying the opposite - that porous bones marine animals must be deep sea divers.

(B) In most modern marine reptile species, the outer shell of the bones is not porous.

We are talking about deep sea divers. Irrelevant.

(C) In most modern and prehistoric marine reptile species that are not deep divers, the outer shell of the bones is porous.

This tells us that porous bones are found in non deep sea divers too. So porous bones does not necessarily mean deep sea divers. This weakens our conclusion.

(D) In addition to the porous outer shells of their bones, whales have at least some characteristics suited to deep diving for which there is no clear evidence whether these were shared by ichthyosaurs.

Irrelevant. We are establishing the connection between porous bones and deep sea diving only.

(E) There is evidence that the bones of ichthyosaurs would have been light enough to allow surfacing even if the outer shells were not porous.

Our argument does not say that porous bones are necessary for deep sea diving. It says that they are sufficient. It says that porous bones implies deep sea divers. Other things could also lead to surfacing after deep sea diving. This would weaken our conclusion if we were concluding that porous bones are necessary for deep sea diving. But we are not concluding that. Hence not correct.

Answer (C)