Final answer:
Option d, undertaking an activity whenever the marginal benefit exceeds the marginal cost, best reflects rational decision-making behavior.
Step-by-step explanation:
Rational decision-making behavior is best reflected by option d. undertaking an activity whenever the marginal benefit exceeds the marginal cost. This means that a rational individual will choose to undertake an activity as long as the additional benefit gained from doing so is greater than the additional cost incurred. By considering the marginal benefit and cost, individuals can make informed decisions based on maximizing their overall well-being or utility.