The phrase "even her harshest critics were temporarily silenced" indicates the speech possessed significant power and persuasive capability.
Ambiguous: Lacking clarity or admitting multiple interpretations, this would more likely elicit further questions or dissent, rather than silence.
Inflammatory: Designed to incite anger or violent emotions, this might lead to outcry or shock, but not necessarily thoughtful silence from critics.
Compelling: Possessing the ability to strongly evoke interest, attention, or admiration, and to be powerfully convincing. A compelling speech could indeed render critics temporarily speechless due to its potency and persuasive force.
Apathetic: Exhibiting a lack of interest, enthusiasm, or concern. An apathetic speech would be unengaging and unlikely to silence critics; it might rather bore them.
Consequently, "compelling" is the most appropriate descriptor.