The Himalayas are formed of parallel fold ranges, of which the oldest range is

The Himalayan system is divided into distinct parallel lithotectonic ranges — Trans (Tethys) Himalaya, the Greater (Himadri) or Great Himalayan range, the Lesser Himalaya (Himachal) and the Shiwaliks — with the Great/Himadri forming the central, oldest axial fold among the principal Himalayan fold ranges [1]. Geological uplift history indicates that the Lesser Himalaya and its spurs (Pir Panjal, Dhauladhar, etc.) were uplifted later during subsequent phases of Himalayan orogeny, making them younger than the central Great Himalaya [2]. Standard competitive-exam keys also record the Great Himalayan Range as the oldest of the parallel fold ranges of the Himalayas.

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 2: Physiography > THE HIMALAYAN REGION > p. 1
2. [2] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 2: Physiography > Fig. 2.6 Northward Drift of the Indian Subcontinent [Source: Himalaya Formation www.usgs.org (US Government)] > p. 8