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

The correct option is Gomati - Ghaghara - Gandak - Kosi.

To understand why this sequence is correct, we can analyze the geographical path of the Ganga river as it flows eastward and identify the locations where its major Himalayan tributaries join it downstream of Prayagraj (Allahabad).

Moving from West to East (left to right on a standard map) along the Ganga river after Prayagraj, the tributaries join in the following order:
1. Gomati: The Gomati river originates in the plains of Pilibhit, Uttar Pradesh, and joins the Ganga near Ghazipur, which is situated just east of Varanasi.
2. Ghaghara: The Ghaghara river (also known as Karnali in Nepal) flows southeast through Uttar Pradesh and Bihar, joining the Ganga near Chhapra in Bihar, further east of Varanasi.
3. Gandak: The Gandak river originates in the Himalayas of Nepal, enters the plains of India, and joins the Ganga near Sonpur/Patna in Bihar, which lies east of Chhapra.
4. Kosi: Known as the "Sorrow of Bihar", the Kosi river drains eastern Nepal and joins the Ganga near Kursela in the Katihar district of Bihar, which is located much further to the east near the West Bengal border.

Therefore, looking at their confluence points with the Ganga from West to East, the correct sequential order is Gomati, followed by Ghaghara, then Gandak, and finally Kosi.