The correct answer is option 'A': Tapi River.

The Ukai Dam is one of the largest reservoirs in India and it is built across the Tapi River. Let's explore more details about the Ukai Dam and the Tapi River.

About the Ukai Dam:
1. The Ukai Dam, also known as the Vallabh Sagar Dam, is located near Ukai village in Gujarat, India.
2. It is built across the Tapi River, which flows through the states of Maharashtra and Gujarat.
3. The dam was constructed to fulfill the water requirements of the region, including irrigation, drinking water supply, and industrial use.
4. It is a multipurpose project that serves various purposes like hydroelectric power generation, flood control, and water supply.
5. The dam has a height of 77 meters and a length of 492 meters. Its reservoir has a storage capacity of 46,500 million cubic feet.

Importance of the Tapi River:
1. The Tapi River is one of the major rivers in central India and flows through the states of Maharashtra, Gujarat, and Madhya Pradesh.
2. It originates in the Betul district of Madhya Pradesh and flows westward before joining the Arabian Sea near Surat in Gujarat.
3. The river has a total length of around 724 kilometers and covers a drainage area of about 65,145 square kilometers.
4. The Tapi River is an important water resource for the states it passes through, providing water for irrigation, drinking, and industrial purposes.
5. It supports the agricultural activities in the region and is a lifeline for the people residing along its banks.

Impacts of the Ukai Dam:
1. The construction of the Ukai Dam has had several positive impacts on the region. It has helped in mitigating floods, as the reservoir can store excess water during heavy rainfall.
2. The dam has also facilitated irrigation in the surrounding areas, leading to increased agricultural productivity and improved livelihoods for farmers.
3. The hydroelectric power generation capacity of the dam has contributed to the energy requirements of the region.
4. Furthermore, the reservoir has become a popular tourist destination, attracting visitors with its scenic beauty and boating facilities.

In conclusion, the Ukai Dam is built across the Tapi River in Gujarat, India. It serves multiple purposes like irrigation, power generation, flood control, and water supply to the region. The Tapi River itself is an important water resource for Maharashtra and Gujarat, supporting agriculture and other activities along its course.