The correct answer is option 3: Source of Hindu Philosophy. The Upanishads are a collection of ancient Sanskrit texts that form the foundation of Hindu philosophy. They are considered the final part of the Vedas, which are the sacred scriptures of Hinduism. The Upanishads explore profound spiritual and philosophical concepts, focusing on topics such as the nature of reality, the self, and the ultimate truth. They contain deep insights into the nature of existence and provide guidance for individuals seeking self-realization and spiritual enlightenment. While the Upanishads contain stories and teachings, they are not primarily storybooks or epics. They also do not function as law books. Rather, they serve as a philosophical guide for seekers of wisdom and spiritual understanding.