How can you create a custom web app for ArcGIS Enterprise?

Creating a custom web app for ArcGIS Enterprise involves utilizing tools and frameworks specifically designed for web application development in the ArcGIS ecosystem. The option highlighting the use of ArcGIS Web AppBuilder or custom JavaScript API development is correct because these tools are integral to designing and deploying tailored applications that interact with the capabilities of ArcGIS Enterprise.

ArcGIS Web AppBuilder provides a user-friendly interface for creating web applications through configurable widgets, allowing users to build apps without deep programming knowledge while still enabling more advanced customizations through the use of custom widgets and themes. On the other hand, utilizing the custom JavaScript API allows developers to create more sophisticated applications that can implement unique functionalities or interfaces that are not available through the Web AppBuilder.

The other approaches mentioned, such as using ArcGIS Pro alone or exclusively modifying existing templates, lack the flexibility and customization options required for developing a fully unique web app. Similarly, relying solely on third-party software would not effectively leverage the built-in capabilities of ArcGIS Enterprise, which are crucial for optimal performance and integration within the ESRI ecosystem. Thus, the combination of Web AppBuilder and the JavaScript API captures the essence of how custom web apps can be efficiently created in ArcGIS Enterprise.