. . . . . . . . specifies the function that will be called when a successful re-connection is completed.

The correct answer is A. callback.

A callback is a function that is called when a certain event occurs. In the context of reconnection, a callback is a function that is called when a successful reconnection is completed.

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 a connection is established.
• D. reconnect is a keyword that is used to initiate a reconnection.

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