In the past earths sea level has sometimes been lower at that time which of the following would have been a major difference in the ocean basins

The answer which would have been a major difference in the ocean basins is the following one:

B. Mid-ocean ridges would have been chains of mountains.

Given that the waters were lower in the past, those ridges would no longer be just mid-ocean ridges, but complete mountains due to the low levels of water around them.