Which feature is typically characterized by underwater volcanic mountains?

Seamounts are underwater volcanic mountains that rise from the ocean floor but do not reach the water's surface. They are formed by volcanic activity, and their shape can vary from steeply conical peaks to flat-topped structures known as guyots. The features of seamounts can provide crucial habitats for marine life and are often associated with rich biodiversity due to the upwelling of nutrient-rich waters around them.

Abyssal plains are flat areas of the ocean floor, typically found at great depths, and they are not volcanic. Continental slopes are the slopes between the continental shelf and the deep ocean floor, and while they can lead to underwater features, they do not specifically refer to volcanic structures. Trenches are deep, narrow depressions in the ocean floor, often formed by subduction of tectonic plates, but they are also not volcanic mountains. Thus, the defining characteristic of seamounts as underwater volcanic mountains makes them the correct answer.