The correct answer is option 3: Narmada. Mahishmati, an ancient city, was located on the banks of the Narmada River. The Narmada River is one of the longest rivers in central India, flowing westward through the states of Madhya Pradesh, Maharashtra, and Gujarat. The location of Mahishmati on the banks of the Narmada River made it strategically important for trade and navigation.

Option 1: Sarayu is not the correct answer. The Sarayu River is a river in northern India, associated with the city of Ayodhya.

Option 2: Son is also not the correct answer. The Son River is a major river in central India, flowing through the states of Chhattisgarh, Jharkhand, and Bihar.

Option 4: Godavari is not the correct answer. The Godavari River is the second-longest river in India, flowing through the states of Maharashtra, Telangana, Andhra Pradesh, and Odisha.

To summarize, Mahishmati was located on the banks of the Narmada River, providing it with strategic significance for trade and navigation in ancient times.