Which type of glacier is known to smooth and round mountains as it flows over them?

The correct answer is the type of glacier known as a continental glacier. Continental glaciers, also referred to as ice sheets, are massive and extensive sheets of ice that flow over land areas, covering wide regions and influencing the landscape significantly. As these glaciers move, their immense weight and slow movement allow them to erode the terrain beneath them effectively.

The process of erosion is key to how continental glaciers smooth and round mountain ranges and hills. The movement of the ice scrapes and plucks rock and soil from the terrain, reshaping the land into a more rounded and polished appearance. This contrasts with valley glaciers, which are smaller and typically confined to mountain valleys and tend to create sharper, more distinct features such as peaks and ridges rather than smoothing them out.

Despite the other options presented, they do not fit the characteristics of glaciers. Laccoliths are geological formations caused by volcanic activity where magma intrudes between layers of rock, and inactive volcanoes do not relate to the processes of glaciation or landscape smoothing. Thus, the defining traits of a continental glacier in shaping and evolving the landscape make it the correct answer to this question regarding glacial impact on mountainous regions.