The line: virtual double function \( X() \) const \( =0 \); in a class definition indicates that the class is probably a: Base class. Library class. Protected class. Derived class.

In C++, the line `virtual double function X() const =0;` signifies that the class is defining a pure virtual function. This indicates that the class is an abstract class, meaning it cannot be instantiated on its own and is likely intended to serve as a base class for other derived classes. Derived classes must provide an implementation for the pure virtual function to be instantiated. Abstract classes are foundational in object-oriented programming, acting as blueprints for other classes. They allow you to define a common interface for a group of related classes while enforcing that certain methods must be overridden. This promotes code reuse and a cleaner architecture, ensuring that all derived classes adhere to a specific contract.