Final answer:
Gibbon and Diderot agree that altruism, which is selflessly caring for the well-being of others, is a necessary duty to promote human happiness.
Step-by-step explanation:
The philosophers Edward Gibbon and Denis Diderot both uphold the belief that a necessary duty of man to contribute to the happiness of the human race is B) Altruism. Altruism, as defined in various scholarly works, is the belief in or practice of disinterested and selfless concern for the well-being of others. Altruism is a form of prosocial behavior that goes beyond merely feeling good about oneself; it encompasses selfless helping of others, even at a personal cost.
In referencing both Gibbon and Diderot's ideals, it's clear that these thinkers advocate for contributing to the happiness of the human race through actions that prioritize collective well-being over individual self-interest. This view aligns with the philosophical concept of the social contract, which implies moral and political obligations to society and encourages behaviors that support the common good.