The correct answer is option 4, Nilgiri Hills. Eucalyptus trees are found in abundance in the Nilgiri Hills. The Nilgiri Hills are located in the Western Ghats of India, spanning across the states of Tamil Nadu, Karnataka, and Kerala. These hills are known for their rich biodiversity and diverse flora and fauna. The cool and moist climate, along with the fertile soil, provides ideal conditions for the growth of Eucalyptus trees. These trees are native to Australia but have been widely planted in various parts of India, including the Nilgiri Hills. Eucalyptus trees are valued for their timber, as well as for the essential oils extracted from their leaves. They are commonly used in the production of paper, construction, and medicinal purposes.