Premises:
The floor of the Black Sea has deposits only of FRESHWATER CREATURES, implying that -- at one time -- the Black Sea was composed of fresh water.
The sediment just above the floor contains the remains only of SALTWATER CREATURES, implying that -- at some point -- the Black Sea was transformed from a freshwater body to a saltwater body..

Conclusion: Seawater from the Mediterranean Sea overflowed into the Black Sea basin, transforming the Black Sea from a freshwater body to a saltwater body..

For evaluate CRs, rephrase the answer choices as STATEMENTS.
The rephrase of the correct answer choice will either STRENGTHEN or WEAKEN the conclusion.

Answer choice E, rephrased:
The remains of the saltwater creatures found in the sediment closely resemble the remains of Mediterranean saltwater creatures from the same period.
This rephrase establishes a link between MEDITERRANEAN SEA and the SALTWATER CREATURES whose remains have been found in the Black Sea, STRENGTHENING the conclusion that seawater from the Mediterranean Sea overflowed into the Black Sea basin.

The correct answer is E.