Hi Lucia,
You can use the Standard multiple choice question with a simple, no css, eliminator workaround:
https://moodleformulas.org/course/view.php?id=22§ion=64
You can also use the multiple choice option in the Formulas question. Two workarounds are proposed: 1) java script; 2) modifying the choice labels. Both workarounds do not require css. The second one requires little coding. Two examples are given for this second workaround: Strikethrough and Gray out. You could modify the XML by replacing the name of the planets with your choices:
https://moodleformulas.org/course/view.php?id=22§ion=8
Also, if you have not already done so, you should vote for this issue that actually you raised: