You are asked to design an API using Flask that will be consumed by multiple client applications with varying security requirements. How would you approach the design to ensure security and flexibility?

To ensure security and flexibility when designing an API for multiple clients with varying security requirements, it's crucial to implement robust authentication and authorization mechanisms. Each client should have its authentication method, and access to sensitive data should be protected. Sharing a single authentication token for all clients is a security risk, and storing sensitive data in plain text or hardcoding client-specific configurations can lead to security vulnerabilities.