Which is used to define the member of a class externally? a. : b. :: c. # d. !!$

b. ::

The scope resolution operator `::` is used to define a member of a class externally in C++. This allows the definition of a member function or a static data member to be separate from its declaration in the class definition.

For example:

class MyClass {
public:
    void myFunction(); // declaration
    static int myStaticVariable; // declaration
};

// definition of myFunction
void MyClass::myFunction() {
    // function body
}

// definition of myStaticVariable
int MyClass::myStaticVariable = 0;

In the example above, `myFunction` and `myStaticVariable` are declared in the class definition, but their definitions are provided externally using the scope resolution operator `::`. This allows the implementation of the member functions and static data members to be separated from the class definition, which can make the code easier to manage and understand.