The correct option is A impromptu
Impromptu means something that is done without being planned or rehearsed, hence a speech made without preparation or rehearsal is impromptu, hence Option A is the correct answer.
Option B is incorrect because “oration” means to publicly speak, which doesn’t necessarily mean it is spontaneous or without preparation, hence incorrect.
Option C is incorrect because “elocution” means the skill of clear and expressive speech, especially of distinct pronunciation and articulation, which is not spontaneously done, hence incorrect.
Option D is incorrect because “declamation” means a rhetorical exercise or a set speech which means a speech that has been prepared for, hence incorrect.