Which principle allows objects to be manipulated as instances of their parent class?

The principle that allows objects to be manipulated as instances of their parent class is polymorphism. Polymorphism enables a single interface to represent different underlying forms (data types). In object-oriented programming, this means that a method can operate on objects of different classes as long as they are derived from the same parent class.

Through polymorphism, one can invoke methods or properties defined in a parent class from instances of various child classes. This is particularly powerful when designing systems that can be extended or modified without altering existing code, allowing for greater flexibility and code reuse.

For instance, if you have a parent class called "Animal" and subclasses like "Dog" and "Cat," polymorphism allows you to treat both dogs and cats as instances of Animal when invoking methods that are defined in the Animal class. This not only simplifies code but also enhances its maintainability, as new animal types can be introduced without modifying existing code structures.