Final answer:
The early factories were mostly built in the Northeast region of the United States due to the availability of labor and markets.
Step-by-step explanation:
The early factories in the United States were mostly built in the Northeast region.
Factories in this region were close to reserves of labor and markets for manufactured products, making it a strategic location. The steel industry, for example, was located in Pittsburgh due to its access to raw materials, while textile manufacturing was centered in New England because of the abundant water sources for power.
Thus, the correct answer is Northeast.