Explanation:

Rigveda

The Rigveda, one of the oldest texts in the world, provides significant insights into the early Vedic period. It contains hymns and verses that offer glimpses into the society, culture, religion, and lifestyle of the people during that era.

While archaeological excavations and contemporary cultures can provide additional information about the early Vedic period, the Rigveda serves as a primary and foundational source for understanding this period in ancient Indian history. Therefore, option 3 (Rigveda) is the most appropriate choice.