Final answer:
The firm is experiencing decreasing returns to scale.
Step-by-step explanation:
The firm is experiencing returns to scale. Returns to scale refer to the change in output that occurs when all inputs are increased in the same proportion. In this case, as the firm increases its production from 2,000 units to 2,100 units, the total cost increases from $8,000 to $8,600. This is an increase in cost, indicating that the firm is experiencing decreasing returns to scale.