Preselected default values
When the Do not preselect default value box is checked on your product options page, a customer can't add the product to their cart without selecting one of the available options first.
When the Do not preselect default value box is unchecked, the default product option is preselected and added to the customer's cart. This makes the checkout process faster.
You can use the Do not preselect default value option with drop-down lists, size, and radio button input types [link to product options].