When users need to select only one option out of a set of mutually exclusive choices.
When the number of available options can fit onto a mobile screen.
When to consider something else
Consider checkboxes if users need to select more than one option or if there is only one item to select.
Consider a dropdown if you don’t have enough space to list out all available options.
If users should be able to select zero of the options.
How to use
Users should be able to tap on or click on either the text “label” or the radio button to select or deselect an option.
Options that are listed vertically are easier to read than those that are listed horizontally. Horizontal listings can make it difficult to tell which label pertains to which radio button.
If you customize, make sure selections are adequately spaced for touch screens.
Use caution if you decide to set a default value. Setting a default value can discourage users from making conscious decisions, seem pushy, or alienate users who don’t fit into your assumptions. If you are unsure, leave nothing selected by default.
Radio buttons typically appear inside of <fieldset>s. The class name of usa-input-error may be placed on the <fieldset> that contains all of the radio buttons.
The error message is placed just below the <legend>.