Final answer:
Plentiful and essential products and services typically have a low marginal benefit because the added value of consuming an additional unit decreases as supply meets demand. This is consistent with the principles of a perfectly competitive market where price equals the marginal cost, indicating a balance between social costs and benefits.
Step-by-step explanation:
Products and services that are plentiful and essential will typically have a low marginal benefit. This is because as a product becomes more abundant and essential, the value of consuming an additional unit or employing one more service is less since basic needs are already met. Therefore, the correct answer is D. a low marginal benefit. In a perfectly competitive market, prices tend to equal the marginal cost of production, aligning the social costs of producing a good with the social benefits received from it. Thus, when products and services are plentiful, most consumers receive less marginal utility as they consume additional units, reflecting a low marginal benefit. A rational consumer, like Alphonso in the provided example, only purchases additional units of a product as long as the marginal utility exceeds the opportunity cost, leading to a decrease in marginal utility and eventually reaching a point where it is no longer worth purchasing more.