Which is the westernmost tributary of the Ganga River?

The correct answer is Option (2) → Yamuna River

The Yamuna rises farthest to the west among these rivers and joins the Ganga at Prayagraj. The Gomti, the Gandak and the Kosi join the Ganga further east.

Note: This question is not from NCERT. NTA does this once in a while, but such questions are very few. Your NCERT is still enough.