Explanation:

- Valmiki:

- Valmiki is the sage who authored the epic 'The Ramayana'.

- Before becoming a sage, he was a robber known as Ratnakar.

- Correct Answer: Valmiki.

- Durvasa:

- A powerful and short-tempered sage.

- Known for his curses and is connected to various other Indian epics but not as Ratnakar.

- Gautam:

- An ancient sage known for his wisdom.

- Known for his wife Ahalya's story, but not synonymous with Ratnakar.

- Vishvamitra:

- A revered sage, formerly a king.

- Played a role in 'The Ramayana' but was not known as Ratnakar.