Option 1: One factor is constant -> The Law of Variable Proportions operates when at least one factor of production is kept fixed (constant) while other factors are varied. This is a short-run phenomenon where we cannot change all inputs. For example, if land and machinery are fixed, and we keep adding more labor, we observe three stages: increasing returns, diminishing returns, and negative returns. This happens because the fixed factor becomes relatively scarce compared to the variable factor, leading to less efficient combinations of inputs. If all factors could be varied, we would be studying returns to scale instead. -> correct