To ensure the confidentiality and integrity of data in transit between the sources and the Lambda function, encryption in transit should be implemented. While AWS Systems Manager (B) can be useful for managing EC2 instances, it does not provide encryption for data in transit. A VPN gateway (D) provides secure connectivity but is not the most effective method for securing Lambda function access. Encryption at rest (A) protects data when it is stored, but it does not protect data during transit. The correct approach (C) is to enable encryption in transit using SSL/TLS for all connections to the Lambda function, ensuring that the data is encrypted and protected during transit.