Q.If Total Utility falls when an additional unit of a commodity is consumed, the Marginal Utility of that unit must be:

Marginal utility is the change in total utility caused by consuming one more unit, so the sign of MU can be read directly from whether TU is rising, unchanged, or falling.

Marginal Utility is defined as $MU_n = TU_n - TU_{n-1}$.

• When $TU_n > TU_{n-1}$ (TU rising), $MU_n$ is positive.
• When $TU_n = TU_{n-1}$ (TU unchanged, at its maximum), $MU_n$ is zero — this is the point of satiety.
• When $TU_n < TU_{n-1}$ (TU falling), $MU_n$ is negative — the extra unit actually reduces total satisfaction (e.g., eating one sweet too many causes discomfort).

Since the question states TU falls with the additional unit, by definition $MU_n = TU_n - TU_{n-1} < 0$.

Confusing 'Marginal Utility is falling' (the Law of Diminishing Marginal Utility, which is about the rate of MU decreasing while MU can still be positive) with 'Marginal Utility is negative' (which specifically means TU itself has started falling). Diminishing MU and negative MU are not the same stage.