What occurs if a rule is checked out but another developer attempts to use it?

When a rule is checked out by a developer in Pega, it indicates that this version is being modified and is not yet saved for others to use. However, if another developer tries to access that same rule while it is checked out, the application will reference the last saved version of the rule instead of the checked-out version. This allows the application to continue functioning without disruption, ensuring that users can work with the most recently available version of the rule.

This mechanism is designed to facilitate collaboration among developers, allowing them to work on rules without creating conflicts or bottlenecks. The application's ability to revert to the last approved version ensures continuity in functionality while changes are being made. Hence, option B correctly describes this behavior.