What Salesforce technology securely exposes an external system User Interface from within Salesforce?

The correct choice highlights the Canvas technology, which is specifically designed for securely integrating external web applications into the Salesforce interface. Canvas provides a unique way to incorporate an external user interface into Salesforce without compromising security. It allows for deep integration by enabling single sign-on and seamless interaction between the external application and Salesforce data.

With Canvas, developers can leverage both Salesforce's security protocols and the capabilities of the external application, ensuring that users have a cohesive experience. Notably, Canvas includes support for OAuth, which helps in maintaining secure transactions and user data integrity during interactions.

Other options, while useful for customizing and enhancing the Salesforce user experience, do not provide the same level of integration and security as Canvas. Visualforce is primarily used for building custom user interfaces but does not inherently integrate external applications. Custom Web Tabs allow for displaying external web content but lack the secure authentication features that come with Canvas, making them less suitable for sensitive data transactions. Lastly, Lightning Components are geared toward creating dynamic web applications within Salesforce, but they do not enable the same level of direct integration with external user interfaces as Canvas.

By leveraging Canvas, organizations can ensure that users access external systems securely within the Salesforce environment, which is pivotal for maintaining data security and providing a streamlined user experience.