Answer:
Letter b is correct. Double but productivity will not change.
Explanation:
Scale returns are characterized by an increase in production associated with an increase in production factors.
It is a concept that relates to the concept of economy of scale, but while in an economy of scale the effect of decreasing unit cost related to an increase in production levels, a return of scale corresponds to the effect of increase that occurs by the relation between the quantity of inputs and production.
Therefore, alternative b is correct, because at a constant return to scale, productivity increases at the same rate as inputs increase.