As output increases, AC curve

Answer: Option D

The average cost is U-shaped because an increase in output increases the returns and reduces the total cost. As the curve continues to slope downwards, it enters a phase of constant returns where the returns and output are at their optimum level. After the constant level, continued increase in output stops yielding any further increments in the returns (diminishing returns) and the costs begin to rise, forcing the curve to start sloping upwards.