Which of the following C++ statements correctly declares an abstract class?

Key point: A class becomes abstract in C++ if it has at least one pure virtual function. A pure virtual function is declared with the virtual keyword and =0.

Correct declaration example:

class A { virtual void show()=0; };

• class A { void show()=0; }; — Invalid: =0 requires the function to be declared virtual. Without the virtual keyword this is ill-formed.

• class A { void show() { } }; — This provides a function body, so the class is concrete (not abstract) and can be instantiated.

• class A { show()=0; }; — Invalid syntax: missing a return type (e.g., void) and the virtual keyword; this is not a valid declaration.

Because the correct declaration contains a pure virtual function, the class cannot be instantiated directly; derived classes must override the pure virtual function to become concrete.