Step 1: Understanding Koshala Mahajanapada.
The Koshala Mahajanapada was an ancient kingdom located in the northeastern part of India. It was situated along the Sarayu River, which is an important river mentioned in ancient Indian texts, particularly associated with the city of Ayodhya.
Step 2: Analyzing the options.
(A) Ganga: The Ganga is a major river in India, but it was not the river associated with the Koshala Mahajanapada.
(B) Yamuna: The Yamuna was not the river associated with the Koshala Mahajanapada either.
(C) Sarayu: Correct. The Koshala Mahajanapada was located on the banks of the Sarayu River.
(D) Gandak: The Gandak is a river in the region, but it was not associated with the Koshala Mahajanapada.
Step 3: Conclusion.
The correct answer is (C) Sarayu, as the Koshala Mahajanapada was situated on the banks of the Sarayu River.