PassMedAnswerHider от Veeloxfire

This extension puts a box over the MCQ answers. That can be removed by clicking a button
This may help practice active recall (as the answers are not there to prompt you)

You can enable and disable this via the extension tab
If you show the answers for a question: this will be remembered while the page and extension is active
(Revisiting a question in a new session or disabling then re-enabling the extension will hide it again)