The most likely region in India to come across the Great Indian Hornbill in its natural habitat is the Western Ghats.

The Western Ghats is a mountain range that runs parallel to the western coast of India, covering the states of Maharashtra, Goa, Karnataka, Tamil Nadu, and Kerala. This region is known for its rich biodiversity and is home to a variety of flora and fauna, including the Great Indian Hornbill.

The Great Indian Hornbill is a large bird with a prominent yellow and black curved bill and a large casque on its head. It is found in the forests of India, Nepal, Bhutan, and Southeast Asia. It is a canopy-dwelling bird and is found in dense forests with large trees.

The Western Ghats is known for its dense evergreen forests, which provide an ideal habitat for the Great Indian Hornbill. The bird is often found in the canopy layer of the forests, where it feeds on fruits, insects, small mammals, and reptiles. The Western Ghats is also home to many wildlife sanctuaries and national parks, such as the Silent Valley National Park, Periyar Wildlife Sanctuary, and Bandipur National Park, which provide protected habitats for the Great Indian Hornbill.

In contrast, the other options listed are not likely to be the habitat of the Great Indian Hornbill. The sand deserts of northwest India are a harsh and arid region, with little vegetation, making it an unsuitable habitat for the bird. The higher Himalayas of Jammu and Kashmir are too cold and rocky, and the bird is not adapted to survive in such harsh conditions. The salt marshes of western Gujarat are a wetland habitat that is not preferred by the Great Indian Hornbill.

In conclusion, the Western Ghats of India is the most likely region to come across the Great Indian Hornbill in its natural habitat due to the dense evergreen forests that provide an ideal habitat for the bird.