Concept:The law of variable proportions operates only when at least one factor of production remains fixed.
Explanation:As increasing quantities of a variable factor are combined with a fixed factor, marginal product first rises, then falls.
This happens because the fixed factor eventually becomes a limiting constraint.
In the short run, at least one factor, such as land or capital, is fixed.
In the long run, all factors can be changed, so no factor remains fixed.
The size of the enterprise, whether small or large, does not determine the applicability of this law.
It is the presence of a fixed factor that makes the law relevant.
Therefore, the law of variable proportions is strictly a short-run concept.
Answer:D. in the short-run period