The correct answer is: C. (A) is true, but (R) is false.
Assertion (A) is true. The law of diminishing marginal utility states that as a consumer consumes more of a good, the additional satisfaction or utility gained from each additional unit of consumption declines. This means that the marginal utility of a good will eventually decrease as the consumer consumes more of it.
Reason (R) is false. Deviation from the situation where marginal utility is equal to price does not necessarily lead to a reduction in maximum utility. For example, if a consumer is consuming a good at a point where marginal utility is greater than price, they could increase their utility by consuming more of the good. However, if a consumer is consuming a good at a point where marginal utility is less than price, they could increase their utility by consuming less of the good.
In conclusion, the assertion is true, but the reason is false.