Final answer:
The optimal action is configuring DLP policies to recognize the secure application as trusted for PII transfer, creating a specific exception without lowering overall security.
Step-by-step explanation:
The best action to allow Personally Identifiable Information (PII) to be shared through a secure application without compromising the organization's security posture is to configure the Data Loss Prevention (DLP) policies to whitelist the application with the specific PII. This means adjusting the DLP system settings to recognize the secure document-sharing application as a trusted entity for transferring PII. It is a targeted exception that addresses the blockage issue while maintaining overall data protection standards, ensuring that only this specific application can transmit PII securely.