Which old idea was most strongly challenged by post-drift studies?

Correct answer: Continents and oceans are permanent and fixed

The direct answer is D, the idea that continents and oceans are permanent and fixed. Post-drift studies examined later evidence related to continental movement, seafloor spreading, ocean-floor formation, magnetic patterns and recycling of crust. This evidence supported a dynamic Earth, where the crust can move and change, rather than an unchanging Earth. Option A, the statement that oceans contain salt, is a basic fact and was not the old fixed-Earth idea being challenged. Option B, the statement that rivers erode, is also a normal process of surface change and is not the main target of continental-drift studies. Option C, the statement that mountains are high, is merely a description and does not express a theory about whether continents and oceans move. Option D is correct because post-drift evidence weakened the belief that continents and ocean basins were permanent, motionless features. In simple steps: old view said continents and oceans stayed fixed; new observations showed crustal movement and renewal; therefore the fixed-continent-and-ocean view was challenged. Exam cue: drift evidence points to a changing Earth and eventually supports plate tectonics.