The correct answer is D because the Model-View-Controller (MVC) design framework is a best practice for developing scalable and maintainable applications on the Salesforce platform. The MVC framework separates the application logic into three layers: the model, which represents the data and business logic; the view, which displays the user interface and user interactions; and the controller, which mediates the communication between the model and the view. By using the MVC framework, developers can modularize their code, reuse components, and avoid tight coupling between layers. The other options are incorrect because they are not design frameworks, but rather development methodologies or architectures. Flux is an architecture for building user interfaces with React, a JavaScript library. Waterfall and Agile are methodologies for managing software development projects, with different approaches to planning, testing, and delivering software. References:
* Apex Developer Guide: MVC Architecture
* Trailhead: Apex Basics for .NET Developers
* Trailhead: Platform Developer I Certification Study Guide