The messageChannel class declares the . . . . . . . . class attribute that defines the key string.

The correct answer is: A. command_key

The messageChannel class declares the command_key class attribute that defines the key string. This attribute is used to identify the message channel.

The command-key and commandkey options are incorrect because they do not include the underscore character. The key_command option is incorrect because it does not start with the letter c.