With reference to the Himalayan rivers joining the Ganga downstream of Prayagraj from West to East, which one of the following sequences is correct?

Here's the west-to-east sequence of the Himalayan rivers joining the Ganga downstream of Prayagraj:

1. Gomati: Originates from Gomat Taal (also called Fulhaar Jheel) near Madho Tanda in the Pilibhit district of Uttar Pradesh. It flows through Sitapur, Lucknow, Barabanki, Sultanpur, and Jaunpur before joining the Ganga near Varanasi.

2. Ghaghara: Originates from the glaciers of Mapchachungo. It collects waters from tributaries like Tila, Seti, and Beri. The Sarda river (also known as Kali or Kali Ganga) joins it before it meets the Ganga at Chhapra.

3. Gandak: Formed by two streams, Kaligandak and Trishulganga. It rises in the Nepal Himalayas between Dhaulagiri and Mount Everest. It enters the Ganga plain in the Champaran district of Bihar and joins the Ganga at Sonpur near Patna.

4. Kosi: An antecedent river originating north of Mount Everest in Tibet, where its main stream Arun rises. After crossing the Central Himalayas in Nepal, it's joined by the Son Kosi and Tamur Kosi, forming the Sapt Kosi. The Kosi joins the Ganga in the Katihar district of Bihar.

Therefore, the correct sequence from West to East is Gomati – Ghaghara – Gandak – Kosi.

Hence option (b) is the correct answer.