Which is considered the longest tributary of the Ganga River?

Correct answer: Yamuna

The Yamuna is generally considered the longest tributary of the Ganga, so option A is correct. It rises near the Yamunotri Glacier in Uttarakhand, flows through the northern plains, and joins the Ganga at the Triveni Sangam near Prayagraj. Its long course and extensive drainage basin make it the best answer among the listed tributaries. Son, Gandak, and Kosi are also important tributaries, but their courses are generally shorter than that of the Yamuna. The question asks specifically about length, not discharge, flood severity, or economic importance. A river may carry more water or cause more flooding without being the longest. The governing reasoning is therefore a comparison of the tributaries’ overall courses.