You are c# developer who is developing a windows application. you need to provide derived classes the ability to share common functionality with base classes but still define their own unique behavior. which object-oriented programming concept should you use to accomplish this functionality?

Polymorphism; the concept of one definition, but multiple implementations.
In C#, this is done using the 'abstract' and 'override' keywords.
Example:

abstract class Shape {
    public abstract double area();
}

class Square : Shape {
    private double size;

    public Square(double size = 0) {
        this.size = size;
    }

    public override double area() {
        return size * size;
    }
}

There are quite a few applicable words here. modeling, programming, etc. let me know the options. 

To expand his vocabulary & proof read