In the given question, we are asked to identify the country that has an unwritten constitution. The correct answer is option 2, the United Kingdom (U.K.).

An unwritten constitution is a system in which the laws and principles of governance exist in multiple sources such as statutes, court decisions, and conventions, as opposed to being codified in a single written document. The U.K. is known for having an unwritten constitution.

Option 1, the United States of America (USA), has a written constitution known as the U.S. Constitution.

Option 3, Pakistan, has a written constitution adopted in 1973.

Option 4, India, also has a written constitution called the Constitution of India enacted in 1950.

Therefore, the correct answer is option 2, the United Kingdom.