Step 1: Understand the question.
We need the country whose constitution inspired India's Fundamental Rights.
Step 2: The source.
The idea of Fundamental Rights came mainly from the United States and its Bill of Rights, which protects civil liberties.
Step 3: What India borrowed.
India took the idea of guaranteed rights enforceable in court, with remedies under Article 32 and Article 226.
Step 4: Rule out others.
The UK gave us the parliamentary system. Ireland gave the Directive Principles. The USSR inspired Fundamental Duties.
Step 5: Conclude.
Fundamental Rights were borrowed from the United States of America.
Answer: United States of America