Multiple Choice questions can be used to display options using either radio buttons or checkboxes.
After adding your Multiple Choice question, click the cogwheel icon to the right to configure it. Toggling on "Can choose more than one answer" will switch the options from radio buttons to checkboxes.
As a shortcut, you can simply type the slash command
/checkbox into a blank line of the editor. This creates a new Multiple Choice question that is already configured for checkboxes.
This is also useful if you need a single checkbox, such as an "I agree" option for a terms and conditions question.