The correct answer is: a) Birdwatching.
Birdwatching is a form of tourism that helps conserve the rich avifauna of Assam. It is a non-consumptive form of tourism that does not harm the environment or the birds. Birdwatchers are often very knowledgeable about birds and their habitats, and they can help to raise awareness about the importance of conservation. They can also provide valuable data on bird populations and habitats, which can be used to develop conservation plans.
River safaris, cultural tourism, and trekking are all forms of tourism that can have a negative impact on the environment. River safaris can damage riverbanks and ecosystems, cultural tourism can lead to the loss of traditional cultures, and trekking can damage fragile ecosystems.
Birdwatching is a sustainable form of tourism that can help to conserve the rich avifauna of Assam. It is a low-impact activity that does not harm the environment or the birds. Birdwatchers are often very knowledgeable about birds and their habitats, and they can help to raise awareness about the importance of conservation. They can also provide valuable data on bird populations and habitats, which can be used to develop conservation plans.