To delegate email sending operations from the Life Sciences mobile app to a third-party service, a consultant must implement the Lsc4ce.ExternalEmailHandler Apex interface. This specific interface is provided by the Life Sciences Cloud managed package to act as a bridge between the application's email functionality and an external email provider. Implementing this interface allows developers to define custom logic for handing off the email payload to the external service, thereby bypassing the standard Salesforce email sending mechanisms and limits.

B. Lsc4ce.MessagingService is a broader interface and not specifically designed for overriding the standard email sending functionality with an external provider.

C. Lsc4ce.EmailQueueProcessor is related to processing email jobs from a queue, but it is not the designated interface for integrating a third-party email delivery service.

1. Salesforce Life Sciences Cloud Developer Guide, "Integrate with an External Email Provider", Section: "External Email Handler Interface". This section explicitly documents the Lsc4ce.ExternalEmailHandler interface and its purpose for third-party email integration.

2. Salesforce Industries, Life Sciences Cloud API Reference, Lsc4ce.ExternalEmailHandler Interface documentation. This reference provides the method signatures and implementation details for using this specific interface.