What best defines altruism?

Altruism is best defined as an unselfish interest in helping others, which reflects a concern for the well-being of other individuals without expecting anything in return. This definition encapsulates the essence of altruistic behavior, highlighting the motivation to assist others purely out of a desire to contribute positively to their lives.

In contrast, the other options focus on self-centered motives or individual interests, which do not align with the core principles of altruism. Self-interest in social relationships emphasizes personal gain, while motivation driven by personal gain or a focus on individual achievement centers on the benefits to oneself rather than the welfare of others. Therefore, the correct answer captures the altruistic spirit of selflessly serving others without seeking personal rewards.