The correct answer is A. demand curve for the factor.
The marginal value productivity curve (MVP) of a factor is a graph that shows the additional output that a firm can produce by using one more unit of that factor. The downward-sloping part of the MVP curve shows the demand curve for the factor. This is because, as a firm uses more of a factor, the marginal product of that factor declines. This means that the firm is willing to pay less for each additional unit of the factor.
The wages of a factor are determined by the intersection of the demand curve for the factor and the supply curve of the factor. The supply curve of a factor shows the quantity of the factor that workers are willing to supply at different wage rates. The intersection of the demand curve and the supply curve determines the equilibrium wage rate.
The level of employment of a factor is determined by the demand for the factor and the supply of the factor. The demand for a factor is determined by the marginal value productivity of the factor. The supply of a factor is determined by the willingness and ability of workers to supply the factor. The equilibrium level of employment is determined by the intersection of the demand curve and the supply curve.
The supply of a factor is not shown on the MVP curve. The MVP curve shows the demand for a factor, not the supply of a factor.