The correct answer is option 4, Tripura. Tripura is a state in northeastern India that is surrounded by Bangladesh on three sides. It shares its borders with Bangladesh on the east, west, and north sides.
Option 1, Nagaland, is a state in northeastern India that shares its border with Myanmar, not Bangladesh.
Option 2, Assam, is also a state in northeastern India. While it shares a small portion of its border with Bangladesh, it is not completely surrounded by Bangladesh.
Option 3, Arunachal Pradesh, is another state in northeastern India that shares its borders with China, Bhutan, and Myanmar. It does not share a border with Bangladesh.
In summary, the state that is completely surrounded by Bangladesh on three sides is Tripura.