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  When I customize the execution modalities (more time, fewer questions) my configuration is not taken into account...

There are two steps to customizing the execution modalities:

When you create the MCQ, configure the personalization per student proposed by default: select the MCQ from the list (colored line) then go to the tab Execution modalities, rubric Personalization per student to configure.

When you link the MCQ to a homework notebook, test or evaluation, configure this personalization again, specifying the students who will benefit from it: select the assignment, test or evaluation from the list (grayed line) go to the tab Execution modalities, rubric Personalization per student and click on No personalization.