The correct answer is impromptu speaking.
Impromptu speaking or an impromptu speech is one where a speech is given spontaneously without any prior planning or preparation. In this instance, where a teacher assigns a student to stand before the class and give a brief speech on effective study habits, it is unlikely that the student assigned had prior knowledge that he or she would be expected to deliver a speech. Thus, the method of delivery used in this situation is impromptu speaking.