The correct answer is B. Constant returns to scale.
Constant returns to scale means that if all inputs are increased by a certain percentage, then output will also increase by the same percentage. In other words, there is no increase or decrease in efficiency as the scale of production increases.
In the given example, the firm triples its input and produces twice the output. This means that the firm is experiencing constant returns to scale.
A. Increasing returns to scale means that if all inputs are increased by a certain percentage, then output will increase by more than that percentage. In other words, there is an increase in efficiency as the scale of production increases.
C. Decreasing returns to scale means that if all inputs are increased by a certain percentage, then output will increase by less than that percentage. In other words, there is a decrease in efficiency as the scale of production increases.
D. Diminishing returns to scale is a special case of decreasing returns to scale. It occurs when the marginal product of an input starts to decrease as more of that input is used.