The theory that states ethical behavior is that which best serves long-term self-interests is:
a. Utilitarianism
Utilitarianism focuses on the outcomes of actions and suggests that the best action is the one that maximizes overall happiness or benefit. In a broader context, it can be associated with long-term self-interests as it encourages actions that lead to the greatest good for the greatest number, which can align with individual self-interests in the long run.
However, it's worth noting that the association of long-term self-interests is often more closely related to the concept of ethical egoism rather than traditional utilitarianism, which emphasizes collective welfare. If you're looking for a theory that focuses explicitly on self-interest long-term, consider ethical egoism, although it's not listed here. But based on the options provided, utilitarianism is the closest fit.