. . . . . . . . specifies the function that will be called when the client disconnects.

The correct answer is A. callback.

A callback is a function that is called when a certain event occurs. In the context of the question, the callback is the function that will be called when the client disconnects.

The other options are incorrect because:

• B. error is a function that is called when an error occurs.
• C. connect is a function that is called when the client connects.
• D. reconnect is a function that is called when the client reconnects.

I hope this helps! Let me know if you have any other questions.