Answer:
D. Diminishing Marginal Returns.
Explanation:
Just as seen in AI's radiator company, it is seen that their is an effect of increasing input in the short run while at least one production variable is kept constant, such as labor or capital. Returns to scale are an effect of increasing input in all variables of production in the long run.
In the other hand, the law of diminishing marginal returns states that with every additional unit in one factor of production, while all other factors are held constant, the incremental output per unit will decrease at some point.