When the total product starts falling, then the marginal product is

Analyzing the question: It asks what happens to marginal product (MP) at the point where total product (TP) begins to fall.

Concept: Marginal product is the extra output gained from employing one more unit of a variable factor (e.g., labour):

MP = change in TP ÷ change in units of labour

So MP is really the slope of the TP curve.

Step by step:

1. While TP is rising at an increasing rate, MP is rising.
2. When TP rises at a decreasing rate, MP is positive but falling (law of diminishing returns).
3. When TP reaches its peak (maximum), MP = 0 (the slope is flat).
4. When TP starts to fall, each extra worker reduces total output, so the change in TP is negative. Therefore MP < 0.

Example:

| Workers | TP | MP | |---|---|---| | 4 | 50 | 10 | | 5 | 54 | 4 | | 6 | 54 | 0 | | 7 | 50 | -4 |

At 7 workers, TP falls from 54 to 50, so MP = -4, which is negative.

Careful distinction: Option E (zero) is the MP when TP is at its maximum. The question says TP is falling, which is after that peak, so MP is negative.

**Answer: D.

Exam tip: Remember "TP up, MP positive; TP peak, MP zero; TP down, MP negative." MP always follows the slope of TP.