The correct answer is D. Law of indifference.
The law of equi-marginal utility states that a consumer will maximize utility by allocating their income in such a way that the marginal utility of each good consumed is equal.
The law of substitution states that consumers will substitute one good for another as the relative prices of the goods change.
Gossen’s first law states that the marginal utility of a good declines as the consumer consumes more of that good.
Gossen’s second law states that consumers will allocate their time and resources in such a way that the marginal utility of each good consumed is equal to the marginal utility of each hour of leisure time.
The law of indifference states that a consumer is indifferent between two bundles of goods if they provide the same level of utility.