The correct answer is: B. Marginal product of a variable factor.
The law of diminishing returns states that in all productive processes, adding more of one input (variable factor) while holding all others constant (fixed factors) will at some point yield lower incremental per-unit returns.
The total product of all resources is the total output produced by all the resources used in the production process. It is calculated by multiplying the quantity of each resource used by its marginal product.
The marginal product of a variable factor is the additional output produced by using one more unit of a variable factor, while holding all other factors constant.
The average product of variable factors is the total output produced by all the variable factors divided by the number of units of variable factors used.
The marginal product of a fixed factor is the additional output produced by using one more unit of a fixed factor, while holding all other factors constant. The marginal product of a fixed factor will eventually decline to zero, as the additional output produced by using one more unit of a fixed factor becomes smaller and smaller.
The law of diminishing returns is a fundamental principle of economics that applies to all productive processes. It is important to understand the law of diminishing returns in order to make efficient use of resources.