. . . . . . . . specifies the function that will be called when there is a new message received from the channel.

The correct answer is: D. callback

A callback is a function that is called when a certain event occurs. In the context of a channel, a callback is a function that is called when a new message is received from the channel.

The other options are incorrect because:

• Reconnect is a function that is called when a channel is reconnected after being disconnected.
• Error is a function that is called when an error occurs while communicating with a channel.
• Connect is a function that is called when a channel is connected.

I hope this helps!