The correct option would be 4. Consider the criticism as a learning experience. This is the only constructive way to receive criticism: it is just another opportunity to improve. Not responding to it is not necessarily an answer, because entering a debate may be constructive. Ignoring it is certainly not advisable. Responding to it with your own criticism would be a defensive attitude. Asking your best friend isn't an option because a friend probably isn't competent enough to judge on the matter, and he or she may support you even though you are wrong.