The Treaty on Integrated Development of Mahakali River was signed between India and Nepal. This treaty aims to facilitate the development of the water resources of the Mahakali River, which is a transboundary river that flows between the two countries.

Option 1, Pakistan, is incorrect as the treaty was not signed between India and Pakistan.

Option 3, Bhutan, is incorrect as Bhutan is not a riparian country of the Mahakali River.

Option 4, Bangladesh, is also incorrect as Bangladesh is not a riparian country of the Mahakali River.

Therefore, the correct answer is option 2, Nepal. The Treaty on Integrated Development of Mahakali River is an important agreement that allows for joint efforts in harnessing the potential of the Mahakali River for the benefit of both India and Nepal.