Final answer:
An antonym for assurance is uncertainty or doubt, while a synonym could be confidence or guarantee.
Step-by-step explanation:
The antonym and synonym for the word Assurance can vary depending on the context in which it is used. Generally, an antonym for assurance could be uncertainty or doubt, implying a lack of confidence or assurance. On the other hand, a synonym for assurance could be confidence or guarantee, both suggesting a certitude or a promise of something.