Answer
Option: d. the slope of the total-product curve in graph (a) is zero.
Work Step by Step
From the figure attached, we can see that the graph of marginal product becomes zero at quantity of labor = 6.5. This is the same point at which the graph of Total Product saturates and further starts decreasing. At quantity of labor = 6.5, $slope$ = $\frac{change\ in\ y\ coordinate}{change\ in\ x\ coordinate}$ = 0 .
Hence, option (d) is correct.
