To answer this question, we need to understand the concept of C# keywords. C# keywords are reserved words that have a specific meaning and cannot be used as identifiers (variable names, method names, etc.) in the code.
Let's go through each option to determine which one is not a C# keyword:
Option A) if - This option is a C# keyword. The "if" keyword is used to perform conditional statements in C#.
Option B) delegate - This option is a C# keyword. The "delegate" keyword is used to declare a reference type that can hold a reference to a method with a specific signature.
Option C) private - This option is a C# keyword. The "private" keyword is an access modifier that specifies that a member (variable, method, etc.) is accessible only within its own class.
Option D) implements - This option is not a C# keyword. The correct answer is D. The "implements" keyword is not a valid keyword in C#. Instead, the "implements" keyword is used in other programming languages like Java to indicate that a class implements an interface.
The correct answer is D. This option is not a C# keyword because the "implements" keyword does not exist in C#.