Researchers in the 1940s found that sediment on the seafloor was thinner than expected. How did this provide evidence for continental drift? Because this seafloor was relatively old, the sediment had turned to rock. Because this seafloor was relatively young, there had been less time for sediment to accumulate. The motion of the continents caused sediment to fall away from the seafloor. The motion of the continents caused sediment to be ground up into smaller pieces.
The last option is the correct answer. Motion of the massive continents and the movement of water over time can grind the sediment into fine particulate. This would register with us as sand or some other material rather than sediment.