Explanation:

3rd option is correct.
Gautam Dharmasutra is the earliest text referring to Sudras.
Gautama Dharmasutra is a Sanskrit text and likely one of the oldest Hindu Dharmasutras (600-200 BCE), whose manuscripts have survived into the modern age.
The Gautama Dharmasutra was composed and survives as an independent treatise, unattached to a complete Kalpa-sutras, but like all Dharmasutras it may have been part of one whose Shrauta- and Grihya-sutras have been lost to history. The text belongs to Samaveda schools, and its 26th chapter on penance theory is borrowed almost completely from Samavidhana Brahmana layer of text in the Samaveda.