Which statement about Blocks in OutSystems reactive apps is correct?

In OutSystems reactive applications, blocks serve as reusable components that can encapsulate a set of UI elements and logic, thus promoting modularity and maintainability within the application. The ability to instantiate blocks on screens and other blocks is a significant feature of the OutSystems platform, allowing developers to create complex interfaces by composing smaller, reusable pieces.

The correct statement emphasizes that blocks can be instantiated not only on screens but also within other blocks. This enables developers to build up an application's user interface hierarchically, layering blocks to provide functionality while keeping the design clean and organized. This promotes reusability and reduces redundancy in code.

The other statements suggest misconceptions about the capabilities of blocks. For instance, the idea that blocks can only be on screens undermines their flexibility. Also, the mention of client actions or external HTML pages does not accurately represent how blocks are utilized within the OutSystems framework; they are specifically designed for use within screens and other components of the application's structure. Each of these points reinforces the correct understanding of blocks and their proper context within the OutSystems platform, ensuring that developers maximize their potential when working with reactive apps.