The correct answer is: B. There is a fixed factor and a variable factor.
The law of variable proportions states that in the short run, when one factor of production is fixed and another factor is variable, there will be a point at which the marginal product of the variable factor will decline.
This is because as more of the variable factor is added, it will be used less and less efficiently with the fixed factor. This will lead to a decrease in the marginal product of the variable factor.
For example, if a farmer has a fixed amount of land, he can increase his output by adding more labor. However, at some point, the additional labor will not be as productive as the previous labor, and the marginal product of labor will decline.
The law of variable proportions is a fundamental principle of economics, and it has important implications for the production of goods and services.
Option A is incorrect because the law of variable proportions can apply to any number of variable factors.
Option C is incorrect because the law of variable proportions applies to the short run, when at least one factor of production is fixed.
Option D is incorrect because the law of variable proportions does not state that variable factors yield less. It states that the marginal product of a variable factor will decline as more of the factor is added.