Invoking methods on string variables in Java is done using the variable name followed by the dot (.) notation, which is the standard syntax for accessing the properties and methods associated with an object. In the case of strings, once you have a string variable, you can call various methods that belong to the String class, such as .length(), .charAt(), .substring(), and many others, directly on that variable.
For example, if you have a string variable named myString, you can get its length by writing myString.length(). This approach follows the principles of object-oriented programming, where methods are called on object instances using the dot notation to enable functionality specific to that object.
The other choices misrepresent how methods can be used with string variables. The first suggests that methods cannot be invoked on variables at all, which is incorrect. The second implies that some special operator is required for different methods, which is not the case; methods are uniformly called with the dot notation. The third choice states that there are no methods available for string variables, which contradicts the well-defined capabilities of the String class in Java.