Question: The Punpun River is a tributary of what trans-boundary river of Asia?

Context paragraphs:
Mahananda River: The Mahananda River (Pron:/ˌməhɑːˈnʌndə or ˌmɑːhəˈnʌndə/) (Nepali: महानदी , Hindi: महानन्दा नदी , Bengali: মহানন্দা নদী ) is a trans-boundary river that flows through the Indian states of West Bengal, Bihar, and Bangladesh.  Its right bank tributary, the Mechi River forms part of Nepal's eastern boundary with West Bengal while the Kankai flows out of Nepal.

Raidāk River: The Raidāk River (also called Wang Chhu or Wong Chhu in Bhutan) is a tributary of the Brahmaputra River, and a trans-boundary river.  It flows through Bhutan, India and Bangladesh.

Punpun River: The Punpun River is a tributary of the Ganges.  It originates in Palamu district of Jharkhand and flows through Chatra, Aurangabad, Gaya and Patna districts of the Indian states of Jharkhand and Bihar.

Mekong: The Mekong is a trans-boundary river in Southeast Asia.  It is the world's 12th-longest river and the 7th-longest in Asia.  Its estimated length is 4350 km , and it drains an area of 795000 km2 , discharging 475 km3 of water annually.

Mechi River: The Mechi River is a trans-boundary river flowing through Nepal and India.  It is a tributary of the Mahananda River.

Ganges: The Ganges ( ), also known as Ganga (] ), is a trans-boundary river of Asia which flows through the nations of India and Bangladesh.  The 2525 km river rises in the western Himalayas in the Indian state of Uttarakhand, and flows south and east through the Gangetic Plain of North India into Bangladesh, where it empties into the Bay of Bengal.  It is the third largest river in the world by discharge.

Piyain River: Piyain River a trans-boundary river of India and Bangladesh.  It is a tributary of the Surma river, which is originates from the Umgat river of Assam.  The river enters Bangladesh through Sylhet district.

Punpun: Punpun is a satellite town in Patna Metropolitan Region in Patna district in the Indian state of Bihar.  It is located 10 km south of Patna.  The name Punpun comes from the river Punpun, a tributary of river Ganges, on whose bank the village has flourished.

Kankai River: The Kankai River (also referred to as Kankai Mai) (Nepali: कन्काई नदी , is a trans-boundary river flowing through Ilam and Jhapa in north to south direction of Nepal.  Holy river of millions of Hindu Devotees living throughout Nepal.  Passes through the hills of Ilam and plains of Jhapa District.  Kotihom river Bank is the centre for Death rituals of millions of Hindus living in Jhapa and Ilam.  Kankai Irrigation Project launched by Nepal government irrigates the southern Jhapa like Shivganj, Pachgachi, Mahavara etc.  Kanki Mai or Mai khola has the several points of tourism like Domukha, Dhanuskoti, Chuli, Chepti and Maipokhari.  The river overfloods during the monsoon and has possibility to overflow through the thousands of hector of fertile plains of Jhapa.  It is really beautiful to view the local fisherman fishing in the river and the children taking bath in heat of sun.  Kotihom Surunga Municipality holds annual Mai Mela Fair where Millions of people from Jhapa, Morang, Ilam and various parts of India come to enjoy and perform the religious activity on the Day of Magh 1st.  Kankai Aryaghat for death rituals in Kotihom bank of the river built recently by the public investment is used for death rituals each day by the people.

Ghaghara: Ghaghara, also called Karnali (Hindi: घाघरा ; "Ghāghrā" [ˈɡʱɑːɡrɑː]; Nepali: कर्णाली ; "Karṇālī" [kʌrˈnɑːliː]; ; "Jiāgélāhé" [t͡ɕi̯ákɤ̌láxɤ̌]) is a perennial trans-boundary river originating on the Tibetan Plateau near Lake Mansarovar.  It cuts through the Himalayas in Nepal and joins the Sharda River at Brahmaghat in India.  Together they form the Ghaghara River, a major left bank tributary of the Ganges.  With a length of 507 km it is the longest river in Nepal.  The total length of Ghaghara River up to its confluence with the Ganges at Revelganj in Bihar is 1080 km .  It is the largest tributary of the Ganges by volume and the second longest tributary of the Ganges by length after Yamuna.

Supporting paragraphs: Punpun River Ganges
Answer: Ganges