The multiple choice prompt type now supports multiple answers.
The updated edit screen lets you select one or more correct answers. Notice the 3 green check marks.
The student view of the same prompt clearly tells them that they can select multiple answers. The circles are also replaced with squares, making it clear they can select/unselect each answer.
The cumulative results and individual results also support the multiple answers. You can see below that every student selected A, but only some of them select B or C.