Marginal product is:

A) the output of the least skilled worker.
B) a worker's output multiplied by the price at which each unit can be sold.
C) the amount an additional worker adds to the firm's total output.
D) the amount any given worker contributes to the firm's total revenue.

Marginal product is the additional output created by adding one more unit of labor while keeping all other input constant. Therefore, Choice C is correct as it refers to the amount an additional worker adds to the firm's total output. The other choices are incorrect as they do not accurately describe the concept of marginal product.