. . . . . . . . Specifies the function that will be called when a successful connection with the PubNub cloud.

The correct answer is A. callback.

A callback is a function that is called when a certain event occurs. In the context of PubNub, a callback is a function that is called when a successful connection with the PubNub cloud is established.

The other options are incorrect because:

• B. error is a function that is called when an error occurs during a connection with the PubNub cloud.
• C. connect is a method that is used to establish a connection with the PubNub cloud.
• D. reconnect is a method that is used to reconnect to the PubNub cloud after a connection has been lost.