Detailed Explanation:

✅ Statement 1 – Correct: Deep gorges form due to intense vertical erosion by rivers cutting through rapidly rising young mountains; the Kali Gandaki Gorge exemplifies this feature of active orogeny.
✅ Statement 2 – Correct: U-turn river courses (sharp bends) occur due to rugged topography and ongoing tectonic uplift forcing rivers to change direction abruptly.
✅ Statement 3 – Correct: Parallel mountain ranges (Greater, Lesser, Outer Himalayas) are characteristic of fold mountain systems formed by compression of sedimentary layers during continental collision.
✅ Statement 4 – Correct: Steep gradients combined with unconsolidated young rocks and active seismic activity make the Himalayas highly prone to landslides.