Final answer:
Marginal product in panel (b) becomes zero when the slope of the total-product curve in panel (a) is zero, indicating the total output has reached its maximum and additional inputs do not increase production.
Step-by-step explanation:
The question pertains to the concept of marginal product as it relates to microeconomics and specifically to the production functions of a business. The marginal product is the change in output resulting from employing one more unit of a particular input, ceteris paribus. But when does the marginal product in panel (b) become zero? It becomes zero when the slope of the total-product curve in panel (a) is zero, which corresponds to the maximum point on the total product curve. At this point, total output is at its peak with the given inputs, and adding one more unit of input will not increase output any further.