The correct answer is: b) Coastal plains.
Eucalyptus trees are an introduced species in Tamil Nadu. They were brought to the state in the 19th century to be used as a source of timber and fuel. Eucalyptus trees are fast-growing and can tolerate a wide range of climates, so they quickly spread to many parts of the state. They are now commonly found in the coastal plains, where they have displaced native vegetation.
The Nilgiris are a mountain range in the Western Ghats. They are home to a variety of plant and animal life, including many species that are found nowhere else in the world. Eucalyptus trees are not native to the Nilgiris and are not commonly found there.
River valleys are also home to a variety of plant and animal life. Eucalyptus trees can be found in some river valleys in Tamil Nadu, but they are not as common as they are in the coastal plains.
Scrub forests are found in dry, hilly areas. Eucalyptus trees can be found in some scrub forests in Tamil Nadu, but they are not as common as they are in the coastal plains.
In conclusion, the correct answer is: b) Coastal plains. Eucalyptus trees are an introduced species in Tamil Nadu and are now commonly found in the coastal plains.