The Brahmaputra, Irrawaddy and Mekong rivers originate on the Tibetan Plateau and initially flow through narrow, parallel mountain gorges. Of these rivers, the Brahmaputra makes a prominent U turn in its course before entering India. This sharp U turn is primarily due to which of the following geological reasons?

Introduction / Context:
In river and structural geomorphology, the course of the Brahmaputra River is an important example of how tectonic forces and mountain building can control drainage patterns. The river flows eastwards in Tibet as the Tsangpo and then takes a dramatic U shaped bend around Namcha Barwa before entering India as the Brahmaputra. This question asks about the main geological reason for that characteristic U turn.

Given Data / Assumptions:
• The rivers mentioned are Brahmaputra, Irrawaddy and Mekong, all originating in or near Tibet. • The focus is on the Brahmaputra making a sharp U turn before entering India. • Options refer to uplift, syntaxial bending and geotectonic disturbances in young folded mountains. • We assume standard understanding of Himalayan structure in school geography.

Concept / Approach:
The Himalayas are a geologically young fold mountain system with complex bending and curving of ranges at their eastern and western ends. These curves are called syntaxial bends. At the eastern syntaxial bend near Namcha Barwa and Gyala Peri, the Tsangpo is forced to curve sharply around the mountain mass and descend southwards into India as the Brahmaputra. While uplift and general tectonic disturbance are background processes, the specific term used in standard explanations of this U turn is syntaxial bending of the young Himalayas.

Step-by-Step Solution:
Step 1: Recall that the Brahmaputra is called the Tsangpo in Tibet and flows eastwards across the plateau. Step 2: Note that at the eastern end of the Himalayas, near Namcha Barwa, the mountain chain bends sharply, forming an eastward and then southward curve known as a syntaxial bend. Step 3: Understand that the river is structurally guided by this bend and forced to take a tight U shaped loop in order to find a path through the mountains. Step 4: Recognise that this specific bending is described in exam oriented literature as syntaxial bending of geologically young Himalayas rather than just general uplift. Step 5: Therefore, among the options, syntaxial bending of the young Himalayas is the most accurate description of the cause of the U turn.

Verification / Alternative check:
Physical geography texts dealing with river capture and structural control of drainage often state that the U turn of the Tsangpo Brahmaputra system is due to the eastern syntaxial bend of the Himalayas. While uplift and geotectonic forces create the mountains in the first place, the term used to explain the pronounced curve is syntaxial bending. Examination keys for similar questions consistently select this option over more general statements about uplift or disturbance.

Why Other Options Are Wrong:
Uplift of the folded Himalayan mountain series is a broad process that formed the Himalayas but does not specifically explain the tight U shaped curve at one particular location. Geotectonic disturbances in Tertiary folded mountain chains is a vague expression and does not mention the distinctive structural bending, so it is less precise than syntaxial bending. Combined effect of uplift and syntaxial bending sounds attractive but exam questions generally look for the key named concept, which is syntaxial bending of the young Himalayas rather than a combination answer.

Common Pitfalls:
Many students choose a very general sounding option such as uplift of the Himalayas or broad geotectonic disturbance, because they know these mountains are tectonic in origin. However, this specific question is testing the technical term syntaxial bend that is highlighted in textbooks. To avoid confusion, remember that the dramatic U turn of the Brahmaputra is directly associated with the eastern syntaxial bending of the Himalayan ranges.

Final Answer:
The sharp U turn of the Brahmaputra is mainly due to the syntaxial bending of the geologically young Himalayas.