Which method is NOT suitable for accessing a flow externally?

The selected answer is the most appropriate choice because accessing a flow through an internal Salesforce dashboard effectively restricts that flow to users within the Salesforce environment. Internal dashboards are designed primarily for internal reporting and visualization within the Salesforce system, limiting external access.

In contrast, embedding using Lightning Out allows flows to be utilized in external applications while maintaining some level of Salesforce platform functionality. Similarly, using Apex and API calls enables developers to integrate flows with external systems through programmatic access, thereby broadening the use cases. Creating a public site link also permits access to the flow from outside the Salesforce platform, making it accessible to anyone with the link.

Thus, of the methods listed, accessing a flow through an internal Salesforce dashboard is the only one that does not facilitate external access, making it the correct choice as the method that is not suitable for accessing a flow externally.