Which of the following river basins is the most populated in the world?

The correct answer is D, Madhya Pradesh. The Son rises at Amarkantak on the Maikal hills in Anuppur district of Madhya Pradesh, the same plateau from which the Narmada starts in the opposite direction. It flows north and then east for about 780 km through Madhya Pradesh, Uttar Pradesh and Bihar, and joins the Ganga near Maner, a little west of Patna. It is the largest right-bank (southern) tributary of the Ganga; its own tributaries include the Rihand and the North Koel, and the Bansagar dam in Madhya Pradesh and the Rihand dam in Uttar Pradesh stand on this system. The famous Koilwar rail-road bridge in Bihar crosses the Son. A is wrong because West Bengal is where the Ganga ends its Indian course, not where the Son begins. B is wrong because Uttar Pradesh only lies on the Son's middle course. C is wrong because Assam belongs to the Brahmaputra basin. Exam tip: Amarkantak sends the Narmada west and the Son north-east; the Son meets the Ganga near Patna.

The correct answer is D, Jhelum. The Kishanganga rises near Gurez in the Kashmir valley, flows north-west through Bandipora, crosses the Line of Control, where it is called the Neelum, and joins the Jhelum at Muzaffarabad in Pakistan-occupied Kashmir. It is the largest tributary of the Jhelum, which itself rises at the Verinag spring and flows through Srinagar and the Wular lake. India's 330 MW Kishanganga hydroelectric project in Bandipora district, opened in 2018, led to a dispute with Pakistan under the Indus Waters Treaty of 1960, which is why this river appears in exams. A is wrong because the Chenab is formed by the Chandra and Bhaga at Tandi in Himachal Pradesh and does not receive the Kishanganga. B is wrong because the Sutlej rises near Rakshastal in Tibet and is fed by the Beas, not the Kishanganga. C is wrong because the Ravi rises in the Kullu hills near the Rohtang Pass. Exam tip: the Kishanganga (Neelum) meets the Jhelum at Muzaffarabad; the Jhelum rises at Verinag; the Indus Waters Treaty was signed in 1960.

The correct answer is B, Darjeeling Hills. The Mahananda rises in the Darjeeling hills of West Bengal, near Kurseong, at a height of about 2,100 metres. It flows down through Siliguri, enters Bihar near Kishanganj, and then passes into Bangladesh, where it joins the Ganga (called the Padma there). It is a left-bank tributary of the Ganga, and its own tributaries include the Balason and the Mechi. The Mahananda Wildlife Sanctuary near Siliguri is named after it, and the river is a reason why Siliguri and the 'chicken's neck' corridor are so important for connecting the North-East. A is wrong because the Chota Nagpur Plateau gives rise to the Damodar and Subarnarekha, not the Mahananda. C is wrong because the Kaimur hills are near the Son valley in Bihar and Madhya Pradesh. D is wrong because Nanda Devi in Uttarakhand feeds the Rishi Ganga and the Alaknanda system. Exam tip: Mahananda = Darjeeling hills, flows through Siliguri, left-bank tributary of the Ganga; Damodar and Subarnarekha = Chota Nagpur.

The correct answer is A, Tamil Nadu. The Arabian Sea branch of the southwest monsoon strikes the Western Ghats first and drops most of its moisture on the windward western slopes, so the land on the eastern, leeward side gets little rain. This dry leeward zone is the rain shadow, and Tamil Nadu lies in it, which is why it receives its main rain later, from the retreating north-east monsoon in October to December. A famous local example is Mumbai getting far more monsoon rain than Pune, which lies just behind the Ghats. B is wrong because Gujarat is on the windward side and gets direct rain from the Arabian Sea branch. C is wrong because Goa lies on the west coast in front of the Ghats and is one of the wettest states. D is wrong because Kerala is where the southwest monsoon first arrives in India, around 1 June, and it gets very heavy rain. Exam tip: the Western Ghats block the Arabian Sea branch; west side wet (Kerala, Goa), east side dry (Tamil Nadu), which is why Tamil Nadu depends on the north-east monsoon.

The correct answer is D, Manipur. Nepal touches five Indian states: Uttarakhand, Uttar Pradesh, Bihar, West Bengal and Sikkim. Manipur lies far to the east in the North-East and shares its international border only with Myanmar. Bihar has the longest border with Nepal, and the India-Nepal border is an open border of about 1,750 km along which people move without a visa. Two easy memory hooks: the Kali river separates Uttarakhand from Nepal, and the Siliguri corridor of West Bengal lies between Nepal and Bangladesh. A is wrong because Sikkim lies between Nepal and Bhutan and shares its western border with Nepal. B is wrong because Bihar has the longest boundary with Nepal among all states. C is wrong because the Darjeeling district of West Bengal borders eastern Nepal. Exam tip: Nepal's five neighbours are Uttarakhand, UP, Bihar, West Bengal and Sikkim; Manipur, Mizoram, Nagaland and Arunachal touch Myanmar.