Final answer:
Mountains across Utah and Nevada were largely formed by tectonic plate movements, with erosion and weathering as well as glacial processes shaping the landscape further.
Step-by-step explanation:
The mountains across Utah and Nevada were primarily formed by tectonic plate movements (Option A). This geological process involves the collision and compression of the Earth's crustal plates, causing the upthrusting and folding of rock layers to great heights, forming extensive mountain ranges. In addition to tectonic activity, other forces like erosion and weathering (Option C) shape the mountains over time by wearing down the high peaks to create sharp, serrated edges. Glacial processes further sculpted the landscapes during past ice ages. Although volcanic activity (Option B) has played a role in shaping the regional terrain, the predominant mechanism of mountain building in Utah and Nevada is due to tectonic forces.