To ensure the confidentiality and integrity of data in transit between the clients and the backend services, SSL/TLS termination should be enabled on the API Gateway. This allows the API Gateway to decrypt incoming traffic and encrypt outgoing traffic, ensuring that the data is protected during transit. Encryption at rest (A) is not applicable to API Gateway, as it does not store data. A VPN gateway (C) provides secure connectivity but does not encrypt the data itself. While enabling encryption in transit (D) is important, SSL/TLS termination on the API Gateway is the most effective method for ensuring the confidentiality and integrity of data in transit between the clients and the backend services.