Option 1 -> Decreasing Returns to scale occurs when doubling inputs results in output increasing by less than double.
Option 2 -> Increasing Returns to scale occurs when doubling inputs results in output increasing by more than double.
Option 3 -> Constant Returns to scale occurs when doubling inputs results in output exactly doubling.
Option 4 -> Decreasing Productivity refers to decline in output per unit of input, not related to proportional input-output changes.
Hence, Option 3: Constant Return to scale -> When all inputs are doubled and output also doubles proportionally, the production function exhibits constant returns to scale. This is a fundamental concept in production theory where f(2K, 2L) = 2f(K, L), meaning the scale of production changes proportionally with inputs. The production function is homogeneous of degree 1 in this case. -> correct