Adam Smith built his productivity argument around the division of labor: when a task is broken into simple repeated steps, workers naturally look for faster ways to do their step, which leads to new tools and machinery. This process of workers and firms improving their methods and equipment is what economists call technological innovation. Ruling out the other choices helps confirm this: factor-price equalization comes from later international trade theory, protectionism was something Smith argued against since he supported free trade, and labor-intensive production by itself just uses more workers without necessarily raising output per worker. \[\boxed{\text{Technological innovation}}\]