Explanation:

An exotic tree, also known as an introduced, alien, or non-native species, is one that has been brought to a new region by human activity, either intentionally or accidentally, and is not indigenous to that area. \nOption A: 'Teak' (Tectona grandis) is a large deciduous tree native to south and southeast Asia, including India. It is highly valued for its timber and is considered an indigenous species in many parts of India. \nOption B: 'Sal' (Shorea robusta) is a species of tree native to the Indian subcontinent. It is a dominant tree in many forests of India and is definitely an indigenous species. \nOption C: 'Neem' (Azadirachta indica) is a fast-growing tree native to the Indian subcontinent and parts of Southeast Asia. It is well-known for its medicinal properties and is an indigenous tree in India. \nOption D: 'Poplar' (Populus species) refers to several species of trees, many of which are native to temperate regions of the Northern Hemisphere. While some species might be found naturally, many Poplar varieties cultivated in India for commercial purposes (e.g., for plywood, matches, paper pulp) are introduced or exotic species, planted extensively in agroforestry systems. Therefore, this option is correct as it is commonly recognized as an introduced species for commercial plantations in India. \nThus, among the given options, Poplar is widely considered an exotic tree species in India due to its extensive introduction for commercial forestry.