How is an asynchronous generator defined in Python?

The correct answer is Option D: Using the async keyword with a regular generator.
In Python, an asynchronous generator is defined by using the async keyword with a regular generator function. This means that you define a generator function using the def keyword, and then decorate it with the async keyword. This combination indicates to the interpreter that the generator function is asynchronous and can be used with asynchronous programming constructs such as await expressions. By using this syntax, you can create asynchronous generators that yield values lazily, allowing for efficient handling of asynchronous data streams.