To declare a class in most programming languages such as Java or C++, the keyword "class" is required.
Let's go through each option to understand why it is correct or incorrect:
Option A) public - The "public" keyword is used to specify the visibility level of class members (variables and methods), but it is not required to declare a class itself.
Option B) private - The "private" keyword is also used to specify the visibility level of class members, but it is not required to declare a class itself.
Option C) class - This option is correct because the "class" keyword is indeed required to declare a class. It is used to define a new class and its associated properties and methods.
Option D) All of the above - This option is incorrect because only Option C (class) is required to declare a class. Options A and B are used to specify the visibility level of class members, but they are not necessary for declaring a class itself.
Therefore, the correct answer is C) class.