Answer D is correct.There actually is a bug in the print_data() function—
$this->$c is interpreted as a variable by PHP, and because the $c variable is not
defined inside the function, no information will be available for printing. Note
that if error reporting had been turned on, either through a php.ini setting or
through an explicit call to error_reporting(), two warnings would have been
outputted instead—but, unless the exam question tells you otherwise, you should
assume that the normal PHP configuration is being used. And in that case, the
interpreter is set not to report warnings.