Keyword used to replace base-class behavior with a derived implementation (polymorphic override) in C#.NET.

Introduction / Context:
Polymorphism requires a base virtual member and a derived override. This distinguishes override from hiding.

Concept / Approach:
Use override in the derived class to supply a replacement for a base virtual/abstract member.

Why Other Options Are Wrong:
new hides; base accesses base implementations; overloads refers to method overloading; overridable is not a C# keyword.

Final Answer:
override