The correct answer is option 'A' - Rig Veda.

Explanation:

The Rig Veda is the oldest and most important of the four Vedas. It is a collection of hymns, chants, and prayers that were composed by ancient sages in India. The Rig Veda is divided into ten books, and each book is called a mandala.

The Rig Veda mentions trade and commerce in several places. It refers to the root Kri (to purchase) and Vanija (to merchants). The Rig Veda also mentions haggling in the market.

Here are some key points about the Rig Veda and its references to trade and commerce:

- The Rig Veda mentions the word "vanij" which means merchant or trader. This shows that commerce was an important part of the Vedic society.
- The Rig Veda also mentions the word "krinu" which means to buy or purchase. This suggests that commercial transactions were taking place in the Vedic period.
- The Rig Veda refers to the concept of "sreni" which means a guild or association of traders. This shows that merchants were organized into groups and had some form of collective bargaining power.
- The Rig Veda also mentions the practice of haggling or negotiation in the market. This shows that commercial transactions were not fixed and that prices were subject to negotiation.
- The Rig Veda mentions different types of goods that were traded, including horses, cattle, and spices. This suggests that there was a diverse range of goods that were being traded in the Vedic period.

In conclusion, the Rig Veda is an important source of information about trade and commerce in ancient India. It shows that commerce was an integral part of Vedic society and that merchants played an important role in the economy.