Answer:
(a) Ghaghara – Gomati – Gandak – Kosi
Explanation:
When considering the sequence of rivers joining the Ganga downstream of Prayagraj from West to East, the correct order is:
Ghaghara joins the Ganga first.
Gomati joins the Ganga after Ghaghara.
Gandak joins next.
Kosi is the easternmost river among these, joining the Ganga after Gandak.
Therefore, the correct sequence is (a) Ghaghara – Gomati – Gandak – Kosi.