Which of the following is true about pure virtual functions?They must be defined in the base class.They can have a definition in the base class.They make a class abstract and cannot be instantiated.They are defined using the keyword pure.

The statement "They make a class abstract and cannot be instantiated" is true about pure virtual functions.

Here's why:

1. Pure virtual functions are declared in a base class and do not have a definition within the base class.

2. They are used to make a class abstract. An abstract class is a class that cannot be instantiated, meaning you cannot create an object of an abstract class.

3. The purpose of an abstract class is to provide an appropriate base class from which other classes can inherit.

4. Pure virtual functions are declared by setting them equal to 0 in the base class, not by using the keyword "pure".

So, the other statements "They must be defined in the base class", "They can have a definition in the base class", and "They are defined using the keyword pure" are not true.