Concept:Returns to scale show how total output responds when a firm changes all inputs by the same proportion in the long run.
Explanation:When a firm doubles all inputs, output should also double if returns to scale are constant.
If output more than doubles, the firm enjoys increasing returns to scale.
If output less than doubles, the firm suffers decreasing returns to scale.
Here, total output rises by less than the doubling of inputs, so this is a case of decreasing returns to scale.
This is different from diminishing returns, which applies when only one input is increased while others are fixed.
Answer:Option D. Decreasing returns to scale.