Explanation:
In Java, access modifiers are keywords used to define the accessibility or visibility of classes, methods, and variables. There are four access modifiers in Java: private, protected, default, and public.

Private:
The private access modifier restricts access to the member only within its own class. It is the most restrictive access level and is often used to protect sensitive information or to encapsulate the internal details of a class.

Protected:
The protected access modifier allows access to the member within its own class, subclasses, and classes in the same package. It is less restrictive than private but more restrictive than default and public. Protected members can be accessed by any subclass of the class in which they are defined, even if the subclass is in a different package.

Default:
The default access modifier, also known as package-private, does not use any keyword. It allows access to the member within its own package but not outside the package. If no access modifier is specified, the member has default access.

Public:
The public access modifier allows access to the member from anywhere, both within its own class and from other classes and packages. It is the least restrictive access level and is commonly used for methods and variables that need to be accessed by other classes.

Invalid Access Modifier:
The correct answer to the question is option 'C' - default. This is because default is not a keyword used as an access modifier in Java. It is a lack of an access modifier, indicating that the member has default access. The absence of an access modifier means that the member is accessible within its own package but not outside the package.

In summary, the four valid access modifiers in Java are private, protected, default, and public. Default is not a separate access modifier but rather the absence of an access modifier, indicating default access within the package.