As an ML engineer at a leading bank, you are tasked with developing a biometric authentication system for the bank's mobile application. The system must verify a customer's identity using their fingerprint, a highly sensitive form of personal information that cannot be stored in the bank's databases due to strict privacy regulations and compliance requirements. Additionally, the solution must be scalable to millions of users worldwide and cost-effective. Considering these constraints, which learning strategy would you recommend for training and deploying this ML model to ensure data privacy and security while maintaining high accuracy? Choose the best option.