Where can a block be used?

A block in OutSystems can be used in multiple contexts, specifically within screens and internal blocks. However, a crucial aspect of blocks is that they cannot reference themselves, which is what makes the correct option standout.

When creating a block, it serves as a reusable component intended to encapsulate functionality or UI elements that can be easily incorporated into different screens or other blocks. This capability enhances modularity and promotes the reuse of code, which is a significant advantage in application development.

The ability to utilize blocks in screens and internal blocks, while excluding self-reference, ensures that developers can design efficiently without running into recursive usage problems, which would lead to complications in execution and maintainability. Thus, the correct option highlights the flexibility of blocks within the outlined constraints, ensuring proper design practices are followed in application structure.