The Brahmaputra, Irrawady and Mekong rivers originate in Tibet and flow through narrow and parallel mountain ranges in their upper reaches. Of these rivers, Brahmaputra makes a "U" turn in its course to flow into India. This "U" turn is due to

Answer: (b) Syntaxial bending of geologically young Himalayas

At the eastern end of the Himalaya the ranges bend sharply around Namcha Barwa (a syntaxis); the Tsangpo follows that bend and turns south into India.

The idea

A syntaxis is a sharp bend where mountain belts curve around the end of a range. A river cutting across a rising belt can follow a deeply incised route through this bend. The question asks for that specific geometry, rather than a general statement that mountains were uplifted.

Why the others are wrong

(a) Uplift alone doesn’t explain a sharp bend.

(c) Too vague — the specific cause is the syntaxial bend.

The trap

(c) sounds technical but describes nothing specific.

Also know

The western Himalayan syntaxis is associated with Nanga Parbat and the Indus gorge. In India, the Brahmaputra’s upper course is called Siang or Dihang before it meets Dibang and Lohit. A meander is a bend made by river erosion and deposition, distinct from a mountain-scale syntaxial bend.