Which of the following was one of the main functions of the Underworld in Greek mythology?

In Greek mythology, one of the primary functions of the Underworld is indeed to hold mortal souls after death. It serves as the realm where the spirits of the deceased reside. The Underworld, often referred to as Hades after its ruler, is depicted as a shadowy place that separates the living from the dead. Upon death, souls are guided to this realm, where they dwell in various sections based on their earthly actions and the choices they made during their lives.

While the Underworld does involve aspects of punishment and reward, such as the severe punishments for wrongdoers and rewards for virtuous individuals in different areas like Elysium, these elements are secondary to its primary role as the destination for all souls. The Underworld does not primarily focus on providing knowledge or wisdom for the living, which further emphasizes that its main function is to serve as a resting place for the souls of mortals.