Exp) Option b is the correct answer.

Pair I is correct: Asmaka or Assaka (near Nanded in the Godavari valley in Maharashtra) is located on the banks of Godavari river between the rivers Godavari and Manjira. The kingdom of Assaka finds mention in a range of texts such as Panini’s Ashtadhyayi, the Markandeya Purana, the Brihatsamhita. Buddhist texts locate Assaka along the Godavari river in Maharashtra. Its capital was Potana/Podana, and is identified with modern Bodhan. The Jatakas suggest that Assaka at some point had come under the sway of Kashi, and that it achieved military victory over Kalinga in eastern India.

Pair II is incorrect: The main river flowing through the Kamboja region is the Jhelum river, also known as Vitasta in ancient texts (and not Vipas river). Kamboja included the present day area of Rajaori, which is in the Hazara district of Pakistan. The Kambojas were a monarchy till about the 6th century BCE, but the later text Arthashastra refers to them as a gana sangha.

• The Vipas river, also known as the Beas River, flows through the northwestern Indian states of Himachal Pradesh and Punjab.

Pair III is incorrect: The mahajanapada of Avanti was located in the Malwa region of central India. Mahanadi river does not flow through the Avanti region. Avanti had two capitals, one at Ujjayini (near modern Ujjain in Madhya Pradesh) and the other at Mahishmati (identified with modern day Mandhata in the western part of Madhya Pradesh). Both the cities were important centres on the trade routes that connected north India with the Deccan and also with the ports on the western coast.

Pair IV is correct: Sarayu was the principal river of the Kosala region (Eastern U.P., included Ayodhya and the tribal republican territory of Shakyas of Kapilavastu). Sarayu river divided the state into two parts: Northern part’s capital: Shravasti and Southern part’s capital: Kushavati Kosala was bound by the Sadanira (modern day Gandak) on the east and the Gomati on the west, the Sarpika or Syandika (Sai) on the south, and the Nepal hills to the north.

Source:

Ancient and Medieval History, Poonam Dalal Dahiya, Chapter – 4, The Mahajanapadas of the Indo-Gangetic Plain (c. 600 – 300 BCE)

https://egyankosh.ac.in/bitstream/123456789/64791/1/Unit13.pdf (Pg. 233 – 235)