Final answer:
Mass manufacturing produces the highest number of parts at the lowest cost, especially when machine hours are cheaper. The selection of production technology depends on the relative costs of labor and machinery, with a preference towards automation when machine costs are lower.
Step-by-step explanation:
The type of manufacturing that produces the highest number of parts at the lowest cost in plants that are specifically designed to produce these parts is Mass manufacturing. Mass manufacturing is optimized to produce large volumes of standardized products, utilizing production lines and automation to reduce the cost per unit. In scenarios where machine hours become more cost-effective, it makes sense for firms to adopt a production technology that relies more on machinery than labor, thus reducing the total cost. This corresponds to the cases where production technology 3, which uses the least labor and the most machinery, is selected due to its lower total cost. However, if the cost of machines increases, there can be a shift towards production technology 2, emphasizing more on labor due to the now relatively higher cost of capital.