Total utility is maximum when marginal utility is zero. This means that when a consumer derives the highest possible satisfaction from consuming a good or service, the additional satisfaction gained from consuming one more unit (marginal utility) becomes zero. Consuming beyond this point does not increase total utility and may even decrease it. In other words, total utility increases as long as marginal utility is positive. When marginal utility reaches zero, total utility is at its peak, and any additional consumption will not add to satisfaction and may reduce total utility if continued further. This relationship is based on the law of diminishing marginal utility, which states that as more units of a good are consumed, the marginal utility from each additional unit decreases. Hence, the total utility is maximum when marginal utility is zero.