The correct answer is option 'C'. A Pandya king sent the earliest embassy, which was met by Augustus at Athens about 20 B.C. This event is recorded by the Greek geographer and historian Strabo in his work "Geographica".

Explanation:

Background:
The history of Tamil Nadu dates back to the prehistoric era, with evidence of human habitation in the region dating back to at least 3000 BCE. The region was ruled by various dynasties, including the Cholas, Cheras, Pandyas, and Pallavas, among others.

Strabo's account:
Strabo was a Greek geographer and historian who lived during the first century BCE. His work "Geographica" provides valuable information about the ancient world, including India. In Book XV, he writes about the Indian subcontinent and the various kingdoms that existed there.

According to Strabo, a Pandya king sent an embassy to Rome, which was received by Augustus at Athens about 20 B.C. The embassy was led by a certain "Kalanos", who is believed to be a corruption of the Tamil word "Kalan", which means "prime minister" or "chief minister".

The purpose of the embassy was to establish trade relations between India and Rome. The Pandya kingdom was known for its maritime trade, and it is likely that the embassy was sent to explore new trade routes and opportunities.

Significance:
The embassy sent by the Pandya king is significant for several reasons:

1. It is the earliest recorded instance of diplomatic relations between India and Rome.

2. It highlights the importance of maritime trade in the ancient world and the role played by South India in facilitating such trade.

3. It provides evidence of the cultural and linguistic connections between ancient Tamil Nadu and the wider world.

Conclusion:
The embassy sent by the Pandya king in 20 B.C. is an important event in the history of Tamil Nadu and India. It highlights the region's role in facilitating maritime trade and the cultural and linguistic connections that existed between ancient Tamil Nadu and the wider world.