Log in to MoodleFormulas, then go to Radio buttons - Several choices.
Correction, you mean
Log in to MoodleFormulas, then go to Multiple Choice -> Radio buttons -> Several choices.
Similar to the OP, I have questions that require the students to select two out of four choices. Students quickly figure out that they just have to select all of the answers to see which ones are the right ones. When they retake the quiz, they simply plug in the right answer.
DO THIS WITHOUT FORMULAS.
I think another way to do this without the Formulas plugin is to use the built-in drag and drop type question.
Create two blanks, with four answers. They have to drag the two correct ones into the boxes.
You could also do this with a fill in the blank, and have them type the letters of the two answers, so if it was B and C, they could type BC.