How do the Upanishads challenge the caste system?

Historically, the most important of the Upanishads are the two oldest, ... Challenges to Brahmanism (6th–2nd century bce) ... The Atharvaveda describes another class of religious adepts, or specialists, the vratyas, particularly ... hierophant (one who manifested the holy) who remained outside the system of Vedic religion.

Explanation:

i am pretty sure it is d.

answer; the responsibility of all person, including civilians and contractors;