What is characterized by crustal plates descending into the mantle?

The option that identifies the characteristic of crustal plates descending into the mantle is associated with deep-ocean trenches. These trenches are essentially subduction zones where one tectonic plate moves under another and sinks into the mantle. This process is crucial in the dynamics of plate tectonics, leading to geological phenomena such as earthquakes and volcanic activity.

Deep-ocean trenches are typically found at the boundaries of tectonic plates, particularly where oceanic crust meets continental crust or where two oceanic plates converge. The immense pressure and heat encountered in these regions facilitate the melting of the descending plate, contributing to the recycling of materials back into the Earth's mantle.

In contrast, seamounts and guyots are underwater volcanoes and flat-topped mountains formed by volcanic activity, which do not involve the descent of crustal plates. Abyssal plains are flat, deep ocean floor areas that result from the accumulation of sediment and are not directly related to subducting plates. Oceanic ridges are areas where tectonic plates are moving apart, leading to the formation of new oceanic crust rather than the subduction characteristic of deep-ocean trenches.