Which exception type occurs when using a variable with no set value (not initialized)?

The NullReferenceException is the correct answer as it specifically relates to attempting to use a variable that has not been initialized, meaning it does not point to any object in memory. When you try to access a method or property of a variable that is null, the runtime raises this exception to indicate that you're trying to dereference a null reference, which leads to an invalid operation.

This is an important aspect of programming, particularly in languages like C# or Java, where variables need to be explicitly instantiated before their members can be accessed. Therefore, handling or checking for null values is a key practice to prevent runtime errors associated with uninitialized variables.

In contrast, the other options refer to different types of exceptions. BusinessRuleException is typically used in scenarios specific to application logic, while SelectorNotFoundException deals with issues relating to UI automation where a specific selector or element couldn't be found. ApplicationException is a more general base class for exceptions that aren't covered by defaults but does not specifically pertain to uninitialized variables.