What natural feature results from volcanic activity?

Volcanoes are direct products of volcanic activity. They form when magma from beneath the Earth's crust rises to the surface, typically through a vent or an opening. This process can occur during explosive eruptions, where lava, ash, and gases are expelled, or during effusive eruptions, where lava flows out steadily. The accumulation of erupted materials creates a structure that we recognize as a volcano.

In contrast, while mountain ranges can also be formed by tectonic activity, they are not exclusively linked to volcanic processes; they can be the result of folding, faulting, or uplift of the Earth’s crust. River valleys, meanwhile, are primarily shaped by erosion and the flow of water over time and are not a direct result of volcanic activity. Fault lines occur due to tectonic movements and are not specifically indicative of volcanic activity either. Thus, the correct answer is that a volcano is the natural feature that results directly from volcanic activity.