Concept:Total utility reaches its maximum when the extra satisfaction gained from consuming one more unit becomes zero.
Explanation:Marginal utility is the change in total utility from consuming an additional unit of a good.
As consumption increases, total utility rises but at a decreasing rate.
At the point of highest total utility, the next unit consumed adds nothing to total utility, so marginal utility is
0.
If marginal utility were positive, total utility would still be rising, so it could not be at its maximum.
If marginal utility were negative, total utility would be falling, meaning the highest level was already passed.
Therefore, the highest level of total utility occurs exactly when marginal utility is zero.
Answer:D. zero