What prefix could you add to the word questionable for it mean not questionable?

There isn't a single perfect prefix to make "questionable" mean "not questionable". However, you can use a few options depending on the specific nuance you want to convey:

* Unquestionable: This is the most straightforward and common option, clearly indicating something that is not open to doubt.

* Indisputable: This emphasizes that the thing is beyond dispute or argument.

* Irrefutable: Similar to "indisputable", it suggests that the thing cannot be refuted or disproven.

* Certain: This focuses on the certainty and reliability of the thing, implying it is not questionable.

* Definitive: This emphasizes the finality and clarity of the thing, leaving no room for doubt.

The best choice depends on the specific context and the desired meaning.