It is not currently possible to set a description for each individual multiple choice question option.
A commonly suggested workaround is to use Conditional Logic and Section Breaks to display text just below the multiple choice question.
We also have a working demo available for this workaround method. You may take a copy by clicking on "Use Template" in the top-right of the linked page.
Here's how to achieve this on your own forms step-by-step:
Navigate to your form editor, and create a new multiple choice or dropdown field.
Enter configuration by hovering over the field, and clicking the cogwheel icon appearing on the right.
Change the options to your liking under the heading "Options", following any relevant on-screen instructions.
Click on "Back to editor" at the top of your configuration window.
Create a new Section Break just below your new field by selecting an empty line, and typing
/section break. A dropdown field will appear, click the option labeled "Section break".
Hover over your new section break and click the cogwheel icon appearing on the right.
Enable the option labeled "Section visibility logic", and pick the new field we just configured from the dropdown menu provided.
Set the middle option to "contains", and the last option to the answer you wish to use with this break. To show this section for multiple answers, just click on "Add another condition" ensuring the new option is set to "Or" not "And".
Go back to your form editor and add the text you want to display for this section after your new section break.
If you'd like to add more sections for different answers, repeat Step 5 → 9 of these instructions.
Once you're finished, repeat Step 5 only to ensure these section breaks don't affect the rest of your form.