The correct answer is: a. The Upanishads
The Upanishads are composed by the writings or sacred texts from the Hindus, these texts talk about God´s nature and the Universe, the meditation and philosophy. They also quote the concepts of charity, non-violence and self-control. The Upanishads are written in sanskrit and are part of the principal texts of the Hinduism written in ancient India (about 2000 years before Christ); they have been transmitted through oral tradition from master to disciple.
The Upanishads are texts with mystical and spiritual reflections from the Vedas; their elevated philosophical tone and deepness made them the base of Hinduism and disciplines such as yoga and meditation. The word Upanishad means to remain seated next to the guru, in order to receive his teachings; it is a connection between the human individual and celestial entities.