Step 1: Identify the role of CAG in India.
The Comptroller and Auditor General (CAG) is India's supreme audit institution; it audits all government accounts and ensures financial accountability of the executive to Parliament; provisions are in Articles 148 to 151.
Step 2: Identify the source country.
India adopted the Westminster model of parliamentary democracy from Britain; the concept of an independent CAG as guardian of the public purse, ensuring parliamentary control over public finances, is directly borrowed from the British system.
Step 3: Eliminate other options.
Russia, Canada, and USA have different audit frameworks; India's CAG is specifically modeled on the British Comptroller and Auditor General institution, making Britain the correct and only answer here.
\[ \boxed{\text{Britain}} \]