The Brahmaputra, Irrawaddy and Mekong rivers originate in Tibet and flow through narrow, nearly parallel mountain ranges in their upper courses. Of these rivers, the Brahmaputra makes a marked U turn to flow into India. This sharp U turn is mainly due to which of the following geological factors?

Introduction / Context:
The Brahmaputra is one of the great Asian rivers with a very unusual course. It flows eastward in Tibet as the Tsangpo, then takes a dramatic U shaped bend near Namcha Barwa to enter Arunachal Pradesh and finally the plains of Assam. This distinctive feature is often examined in physical geography because it reflects the structural arrangement of the Himalayas, especially at their eastern syntaxis where the mountain chain bends sharply.

Given Data / Assumptions:

• Rivers considered: Brahmaputra, Irrawaddy, and Mekong, all originating near the Tibetan Plateau.
• They flow through narrow, parallel mountain ranges in their upper reaches.
• The Brahmaputra alone makes a pronounced U turn before entering India.
• We must identify the main geological reason for this sharp bend.

Concept / Approach:
The Himalayas are not a straight line; they curve in the east and west forming syntaxes, which are points where the mountain ranges bend sharply. The Brahmaputra flows along the northern side of the Himalayas and then turns around the eastern syntaxis near Namcha Barwa. This bend is associated with syntaxial bending of the geologically young Himalayan ranges rather than with an isolated local uplift or random disturbance. Understanding the concept of a syntaxial bend is key to choosing the correct option.

Step-by-Step Solution:
Step 1: Recall that the Brahmaputra flows eastward in Tibet as the Tsangpo along the northern flank of the Himalayas. Step 2: Near Namcha Barwa, the river suddenly turns southward, carving a deep gorge and entering Arunachal Pradesh as the Siang or Dihang. Step 3: This dramatic change in direction coincides with the eastern syntaxis of the Himalayas, where the mountain belt itself sharply bends. Step 4: Syntaxial bending means that the overall structural trend of the mountain range curves, redirecting river courses that follow structural lines. Step 5: While the Himalayas are indeed uplifted folded mountains, the simple uplift does not fully explain the specific U shaped course; the syntaxial bend does. Step 6: Therefore, the primary cause cited in geomorphology is the syntaxial bending of the geologically young Himalayas.

Verification / Alternative check:
A useful check is to compare the courses of the Irrawaddy and Mekong, which also originate in or near Tibet but follow different structural directions without making the same dramatic loop. The unique bend of the Brahmaputra corresponds closely to the easternmost curvature of the Himalayan arc. Geological studies and geomorphological explanations repeatedly highlight this structural control, reinforcing that syntaxial bending is the key factor rather than a random tectonic disturbance.

Why Other Options Are Wrong:
Option A mentions uplift of the folded Himalayan series, which is generally true for the formation of the Himalayas but is too broad and does not specifically account for the sharp U turn. Option C refers to geo tectonic disturbance in Tertiary folded mountain chains, which again is vague and nonspecific. Option D, combining A and B, incorrectly suggests that uplift and syntaxial bending are equally central in this particular context. The precise technical explanation in textbooks emphasises the syntaxial bend, corresponding to option B alone.

Common Pitfalls:
Examinees may be tempted by options that sound more complicated or by those that combine multiple statements, assuming that a mix of causes must be correct. Others may confuse general orogenesis (mountain building) with the specific shape of the range. To avoid these traps, focus on the distinct keyword syntaxial bending, which is consistently linked with the Brahmaputra bend in standard geography references on the Himalayan structure.

Final Answer:
The sharp U turn in the course of the Brahmaputra is mainly due to the syntaxial bending of the geologically young Himalayas at the eastern end of the range.