Which one of the following rivers marks the western boundary of the Ganga Plains ?

The Ganga Plain is the central part of the Great Northern Plains of India. It is geographically defined as the region extending from the Ghaggar river in the west to the Teesta river in the east. The Ghaggar river serves as the western boundary, effectively acting as the transition zone between the Punjab-Haryana Plains and the Ganga Plains.

To the north, the plain is bounded by the Shiwalik range, and to the south, it meets the northern edge of the Peninsular Plateau. Although the Yamuna is the westernmost major tributary of the Ganga and flows along this border, the physiographic limit of the "Ganga Plain" unit is defined by the Ghaggar. The Chambal is a tributary of the Yamuna, and the Luni is a river of the Thar Desert, neither of which defines this specific boundary.