Final answer:
A consumer will purchase a good or service when the marginal utility is greater than or equal to the opportunity cost, which is in line with rational self-interest decision-making.
Step-by-step explanation:
Because consumers behave in a rationally self-interested manner, the consumer will purchase a good or service when the marginal utility is greater than or equal to the opportunity cost. This principle is based on the concept that a rational individual will engage in an action if they perceive the benefits to be greater than the costs. Specifically, a rational consumer weighs the additional satisfaction or utility received from consuming one more unit of a good against what must be given up (the opportunity cost) to acquire that good.
For instance, let's consider Alphonso's choice in purchasing bus tickets. As Alphonso purchases more bus tickets, the marginal utility he derives from each additional ticket diminishes. Concurrently, the opportunity cost, which in this scenario is the marginal utility of the burgers he foregoes, increases. At some point, this opportunity cost will surpass the marginal utility obtained from another bus ticket. If Alphonso is acting rationally, he will cease purchasing more bus tickets once the marginal utility from an additional ticket is equivalent to its opportunity cost.