The odd one out is Bhutan.
Afghanistan, India, and Pakistan are all members of the South Asian Association for Regional Cooperation (SAARC), while Bhutan is not. SAARC is a regional intergovernmental organization that promotes economic, social, cultural, technical, and scientific cooperation among its member states. It was established in 1985 and has its headquarters in Kathmandu, Nepal.
Afghanistan is a landlocked country in Central Asia. It is bordered by Pakistan to the east and south, Iran to the west, Turkmenistan, Uzbekistan, and Tajikistan to the north, and China to the northeast. Afghanistan has a population of over 38 million people and is the 40th most populous country in the world. The capital and largest city is Kabul.
India is a country in South Asia. It is the second-most populous country in the world, with over 1.3 billion people. India is a federal republic with 28 states and 7 union territories. The capital and most populous city is New Delhi.
Pakistan is a country in South Asia. It is the sixth-most populous country in the world, with over 220 million people. Pakistan is a federal republic with four provinces, four territories, and one federal capital territory. The capital and most populous city is Islamabad.
Bhutan is a landlocked country in South Asia. It is bordered by China to the north and India to the south, east, and west. Bhutan has a population of over 750,000 people and is the 135th most populous country in the world. The capital and largest city is Thimphu.
Bhutan is not a member of SAARC because it does not share a land border with any of the other member states. Bhutan is also a relatively small country with a population of just over 750,000 people. This makes it difficult for Bhutan to participate in SAARC activities and programs.