What does the virtual keyword allow when applied to a method?

When the virtual keyword is applied to a method in a class, it allows that method's execution to be overridden in any derived class. This is a fundamental aspect of polymorphism in object-oriented programming. By declaring a method as virtual in a base class, you enable derived classes to provide their specific implementation of that method. This functionality is essential for allowing flexibility and dynamic behavior in applications, as derived classes can customize or extend the behavior of base class methods while preserving a common interface.

This mechanism allows for runtime decision-making in method invocation, where the actual method that gets executed is determined by the object's runtime type rather than its compile-time type. For example, if a base class has a virtual method, and a derived class overrides that method, when you call the method on an object of the derived class, the overridden version will be executed. This is crucial for scenarios such as implementing different behaviors for different types of objects that share a common interface.

The other options do not reflect the purpose of the virtual keyword. For instance, enforcing a method's behavior would imply a lack of flexibility that the term "virtual" inherently provides. Preventing access or enhancing performance are not associated with the implications of method overriding that the virtual keyword promotes.