Final answer:
Altruism is the ultimate and tenderest of prosocial acts, involving selfless care for others' well-being. It is motivated by a desire to help others even if the costs outweigh the benefits of helping. Altruism is exemplified by the heroic actions of individuals like the employee in the World Trade Center during the 9/11 attacks.
Step-by-step explanation:
According to the information provided, the ultimate and tenderest of prosocial acts is altruism. Altruism is the selfless care for others' well-being, motivated by a desire to help others even if the costs outweigh the benefits of helping. It involves disregarding personal costs associated with helping and is considered a pure form of helping others out of empathy. An example of altruism is the heroic actions of an employee in the World Trade Center during the 9/11 attacks who went back into the burning building to help his co-workers even though it resulted in the loss of his own life.