What solution can be implemented if a legacy application does not provide an API, but data needs to be accessed within Salesforce?

The correct solution involves leveraging Canvas to display the legacy application within Salesforce. Canvas enables you to seamlessly integrate external applications right within the Salesforce user interface. This means that even if the legacy application does not provide an API, you can still present its functionality and data within Salesforce.

Canvas acts as an embeddable feature that allows users to interact with the legacy application without leaving the Salesforce environment, providing a smooth user experience. This is particularly useful for applications that may not be easily accessible via traditional API methods but still hold valuable data that needs to be viewed or interacted with by Salesforce users.

The other options do not effectively address the limitation of the legacy application lacking an API. Utilizing the Streaming API is focused on real-time updates and would not help in accessing the data directly from an application that does not expose its data through an API. Migrating the legacy application to Heroku may be a potential long-term solution, but it does not provide a quick way to access the existing data within Salesforce. Creating a web-tab pointing directly to the legacy application might allow access, but it does not offer the integrated experience that Canvas provides, which combines functionality and a user-friendly interface.