The correct answer is option 'A' - Karikala.

Karikala was a legendary Chola king who ruled over the Tamil region during the Sangam period. He is known for his contributions to irrigation and is credited with the construction of the Grand Anicut (Kallanai) on the Kaveri River. The Grand Anicut is an ancient dam built across the river near Tiruchirappalli in Tamil Nadu, India.

Understand the context and background of the question:
During ancient times, the people of the Tamil region heavily relied on agriculture for their livelihood. The Kaveri River, one of the major rivers in South India, played a crucial role in irrigation and farming. However, the river's flow was unpredictable, leading to water scarcity during certain periods.

Identify the key components of the question:
1. The king who built the Grand Anicut on the Kaveri River.
2. The new channel called Vennar.

Explain the significance of the Grand Anicut:
The Grand Anicut, also known as Kallanai, was an engineering marvel constructed by Karikala around the 2nd century CE. It was built to regulate the flow of the Kaveri River and ensure a steady water supply for irrigation purposes. The dam not only controlled flooding but also facilitated the diversion of water into several canals for irrigation.

Discuss the importance of the Kaveri River:
The Kaveri River is considered a lifeline for the people of Tamil Nadu and Karnataka. It is a major source of water for agriculture, drinking, and industrial purposes. The river has been revered in religious and cultural contexts and has played a significant role in the region's history.

Introduce the new channel called Vennar:
The question mentions a new channel called Vennar, through which water from the Kaveri River is drawn off. The Vennar is an artificial channel constructed to divert water from the Kaveri River to the districts of Pudukkottai and Thanjavur in Tamil Nadu. This channel was created to aid irrigation in the region and improve water distribution.

Explain the connection between Karikala, the Grand Anicut, and the Vennar channel:
Karikala, as mentioned earlier, was responsible for building the Grand Anicut on the Kaveri River. This ancient dam served as the foundation for the irrigation system in the region. Over time, additional channels and canals were constructed to distribute water to various parts of the Tamil region. One such channel is the Vennar, which draws water from the Kaveri River and supplies it to the districts of Pudukkottai and Thanjavur.

Conclusion:
In conclusion, Karikala, the legendary Chola king, made the Grand Anicut on the Kaveri River. The water from the river is now drawn off through the Vennar channel, which was constructed to aid irrigation in the Tamil region. The construction of the Grand Anicut and the subsequent development of irrigation systems played a crucial role in the agricultural prosperity of the region.