How do you define a private variable in a Python class?

In Python, a private variable is typically defined by using a double underscore prefix in the variable name. This convention results in name mangling, which helps to avoid name conflicts in subclasses. When you prefix a variable with two underscores, Python changes the name of the variable in a way that makes it harder to access from outside the class. For example, if you define a variable as __my_variable, it will be internally renamed to _ClassName__my_variable, where ClassName is the name of your class. This makes it less accessible directly through an instance of the class, promoting encapsulation and protecting the variable from being modified unintentionally from outside the class.

Using a single underscore prefix is often used to indicate that a variable is intended for internal use (protected), but it does not enforce true privacy. The other options do not correctly represent how to strictly define a private variable in Python. For instance, the 'protected' keyword does not exist in Python, and declaring a variable in a separate private method does not inherently make it private in the sense of visibility; it just controls accessibility through method scope.