The Upanishads are a collection of texts of religious and philosophical nature, written in India probably between c. 800 BCE and c. 500 BCE, during a time when Indian society started to question the traditional Vedic religious order. Some people during this time decided to engage in the pursuit of spiritual progress, living as ascetic hermits, rejecting ordinary material concerns and giving up family life. Some of their speculations and philosophy were compiled into the Upanishads. so the answer would be c.
