Correct Option (b):
The 'U' turn of the Brahmaputra River, also known as the Great Bend, occurs near Namcha Barwa in Arunachal Pradesh. This sharp southward bend is primarily attributed to the syntaxial bending of the geologically young Himalayas in the eastern sector. A syntaxial bend refers to a tight, hairpin-like curve in the structural alignment of mountain ranges, where major tectonic forces cause the fold axes to converge. The Brahmaputra, flowing eastward as the Yarlung Tsangpo in Tibet, is compelled to take this abrupt turn due to this specific geological feature, subsequently entering India.
Incorrect Options:
Option (a): Uplift of folded Himalayan series. While the general uplift of the Himalayas is crucial for the formation of river systems and their gradients, it does not specifically explain the sharp, localized 'U' turn of the Brahmaputra. Uplift influences overall flow patterns and incision, but not the distinct syntaxial bend.
Option (c): Geo-tectonic disturbance in the tertiary folded mountain chains. Geo-tectonic disturbances are broad geological processes responsible for the overall formation and evolution of the Himalayas, which are tertiary folded mountain chains. However, this option is too general to precisely explain the specific phenomenon of the Brahmaputra's 'U' turn, which is more accurately described by syntaxial bending, a particular outcome of these tectonic forces.
Option (d): Both (a) and (b) above. Since option (a) is not the direct and specific cause of the 'U' turn, this combined option is incorrect. The syntaxial bending (option b) is the primary and most accurate geological reason for this distinct river course.