A fossil recently discovered in Marlandia, a chain of islands, proves that a present-day reptile indigenous to Marlandia is descended from an ancient reptile species that lived on the islands millions of years ago. The finding is surprising since the ancestral species was thought to have become extinct when Marlandia was submerged in a global sea-level rise twenty-five million years ago. Based on the new discovery, many scientists have concluded that the sea-level rise in question left at least part of Marlandia unsubmerged.
Which of the following would, if true, provide the most additional support for the scientists' conclusion?
A. Reptiles in Marlandia have adapted to many environmental changes since the sea-level rise.
B. Marlandia separated from a much larger landmass about eighty million years ago.
C. No fossils that prove the relationship between the present-day species and the ancestral species have been found anywhere other than Marlandia.
D. The present-day reptiles are able to thrive on very tiny Marlandia islands.
E. The ancestral reptiles could not have survived long at sea.

E. The ancestral reptiles could not have survived long at sea.  

Explanation:

To get the answer to this question, the key is to focus on which option strengthens the conclusion of the scientists.

The scientists concluded that the sea level rise left at least a part of the Islands unsubmerged which may have caused some of the reptiles to survive. looking at the options, Option A weakens the conclusion of the scientist because if they can adapt, more of the reptiles would have survived in all the islands.

But option E strengthens the conclusion because since the ancestral reptiles could not survive long at sea, it therefore implies that there was at least a part of the island that was not submerged which helped in the survival of a population of the ancestral reptiles during the sea level rise.

Option B is irrelevant and option C is wrong because the statements clearly indicates that fossils shows that the current reptiles  can be traced to the ancestral reptiles on the Island.

The Ancestral reptiles could not have survived long at sea ( E )

Explanation:

From the scientists discoveries and conclusion an additional support to his conclusion would be that the Ancestral reptiles of the present day indigenous reptiles in Marlandia could not have survived long at sea.

From the Fossil recently discovered many parts of Marlandia chain of Islands were submerged in water but the Ancestral reptiles were still able to survive and reproduce the present day reptiles found in Marlandia but scientists believed that they could not have survived long at sea so this brought the new conclusion that at least part of the Islands wasn't submerged and this ancestral reptiles survived on the lands there were not submerged in water.

hence my answer : The ancestral reptiles could not have survived long at sea as an additional support to the conclusion by the scientist.