What became clear from later marine studies?

Correct answer: Ocean floor has features like ridges and trenches

Later marine studies showed that the ocean floor is not a featureless, perfectly flat surface. It contains varied relief, including long submarine mountain chains called mid-ocean ridges, deep depressions called trenches, basins, and other uneven forms. Mapping and studying these features helped scientists understand seafloor spreading, the movement of tectonic plates, and the continuing development of ideas about continental drift. The ocean floor has its own complex geography.

Option C is correct because it identifies ridges and trenches, two major ocean-floor features. Option A is meaningless, option B incorrectly treats the floor as only flat, and option D wrongly describes it as entirely ice. A small flat area does not make the whole seabed flat. Evidence from soundings, mapping, and marine exploration established that the seabed has significant relief, so the selected answer follows directly.