Sonar stands for “sound navigation and ranging.” what assumption was proved wrong after the invention of sonar? the age of the ocean floor is variable. the ocean floor is featureless. the ocean floor is changing. the ocean floor is denser than continental crust.

The assumption that was proved wrong after the invention of sonar is the assumption that THE OCEAN FLOOR IS FEATURELESS.  
The Sonar device was invented during the World War 1 and it is used for accurate mapping of the ocean floor. Before the device was invented, it was assumed that the ocean floor do not contain any feature, but after the invention of the device, it was discovered that, the ocean floor is not flat but have land forms such as mountains and valleys; some of the mountains on the ocean floor are even taller that the ones we have on land while some of the valley are deeper than the one present on land.