Final answer:
The best illustration of reciprocity in helping behavior is when the likelihood of assisting someone is influenced by whether they have previously offered you help, as this exemplifies reciprocal altruism. The correct answer is option B.
Step-by-step explanation:
The observation that best illustrates the act of reciprocity influencing helping behavior is: B. If someone has the opportunity to help you but chooses not to, you are less likely to offer them assistance when they need it in the future. This is because the concept of reciprocity involves a mutual exchange where one's actions are influenced by the actions of others. In the context of prosocial behavior, this mutual exchange implies that we are more inclined to help those who have helped us or are expected to help us in the future. The idea of reciprocal altruism is observed in many species and is based on the expectation of future reciprocation of helping behaviors. This differs from pure altruism, which is selfless helping not motivated by personal gain or the anticipation of reciprocation.