The correct answer is (b). The Himalayan Frontier Thrust (HFT) is a major thrust fault that separates the Lesser Himalaya (Shivaliks) from the Greater Himalaya. The HFT is a reverse fault, meaning that the hanging wall has moved up relative to the footwall. The HFT is thought to have formed during the collision of the Indian and Eurasian plates, which began about 50 million years ago. The collision of these two plates caused the Indian plate to be subducted beneath the Eurasian plate, and the HFT is one of the faults that formed as a result of this subduction. The HFT is a major tectonic feature that has had a significant impact on the geology and landscape of the Himalayas.
The other options are incorrect. Option (a) is incorrect because the HFT lies between the Lesser Himalaya (Shivaliks) and the Greater Himalaya. Option (c) is incorrect because the HFT lies between the Shivaliks and the Gangetic Plain. Option (d) is incorrect because the HFT lies between the Greater Himalaya and the Trans-Himalaya.