Which of the following statement is true? If a base class declares a pure virtual function, a derived class must implement that function to become a concrete class. Referring to a derived-class object with a base- class handle is dangerous. A class is made abstract by declaring that class virtual. All virtual functions in an abstract base class must be declared as pure virtual functions.

1. **Statement:** If a base class declares a pure virtual function, a derived class must implement that function to become a concrete class. **Explanation:** In C++, declaring a pure virtual function in a base class makes the class abstract. An abstract class cannot be instantiated. To create a concrete (instantiable) derived class, the derived class must override the pure virtual function. For example, if the base class has \[ \texttt{virtual void foo() = 0;} \] then the derived class must provide an implementation for \( \texttt{foo()} \). If it does not, the derived class will also be abstract. Therefore, statement 1 is true. 2. **Statement:** Referring to a derived-class object with a base-class handle is dangerous. **Explanation:** In C++, using a base-class pointer or reference to refer to a derived-class object is a core concept of polymorphism and is safe when the base class has a proper virtual destructor and the object is used within the designed inheritance hierarchy. Hence, using a base-class handle is not inherently dangerous. 3. **Statement:** A class is made abstract by declaring that class virtual. **Explanation:** A class becomes abstract when it contains at least one pure virtual function, not simply because the class or its functions are declared as virtual. Declaring the class itself as virtual is not a concept in C++. Thus, this statement is false. 4. **Statement:** All virtual functions in an abstract base class must be declared as pure virtual functions. **Explanation:** An abstract base class can have both pure virtual functions and regular virtual functions with implementations. It only needs at least one pure virtual function to be considered abstract. Therefore, this statement is false. **Conclusion:** The true statement is the first one: If a base class declares a pure virtual function, a derived class must implement that function to become a concrete class.