Correct Option: D Explanation of the Correct Answer: The marginal physical product of labor (MPL) refers to the additional output produced when one more unit of labor (i.e., one additional worker) is employed, while keeping all other factors of production constant. This concept is crucial in understanding how labor contributes to production in an economic context.

1. Understanding Marginal Physical Product (MPL):
2. The MPL is calculated by taking the change in total output (or total product) that results from employing one more worker.
3. Mathematically, it can be expressed as: [ MPL = \frac{\Delta Q}{\Delta L} ] where ( \Delta Q ) is the change in output and ( \Delta L ) is the change in the number of workers (which is typically 1 in this case).
4. Why Option D is Correct:
5. Option D states that the marginal physical product of labor is the additional output achieved when one extra worker is added to the fixed account of other factors. This aligns perfectly with the definition of MPL, as it emphasizes that the output change is due to the addition of labor while other inputs (like capital or land) remain unchanged.

Why the Other Options are Incorrect:

• Option A: "The output of labor unaided by machinery."
• This option is incorrect because it misrepresents the concept of MPL. The MPL does not focus on whether labor is aided by machinery or not; rather, it is concerned with the additional output generated by adding one more worker. The presence or absence of machinery does not define the marginal product of labor.
• Option B: "The extra revenue from the output of one additional worker."
• This option confuses the concept of marginal physical product with marginal revenue product (MRP). While MRP does consider the revenue generated from the additional output produced by an extra worker, MPL strictly refers to the physical output itself, not the revenue derived from it. Therefore, this option is not a correct definition of MPL.
• Option C: "The difference in the output of two workers."
• This option is misleading because it does not accurately capture the essence of marginal productivity. The difference in output between two workers does not necessarily reflect the marginal product of labor, as it could include the effects of diminishing returns or other factors. MPL specifically looks at the output change resulting from the addition of just one worker, not the comparison between two workers.

Summary of Key Points:

• The marginal physical product of labor (MPL) measures the additional output from hiring one more worker while keeping other inputs constant.
• It is calculated as the change in total output divided by the change in labor.
• Option D correctly defines MPL, while the other options misinterpret or confuse the concept with related but distinct ideas.
• Understanding MPL is essential for analyzing labor's contribution to production and making informed decisions about hiring in an economic context.