On this help page is a section called "Special Rules "
http://www.shopsite.com/help/9.0/en-US/ ... tions.html
In there are listed all the attributes that are associated with Order Options. The top one in the table is the 'N' option that says the option is not a valid selection and will require the shopper to make a selection of one of the other options before they can check out of the cart. (It is not required to add the product to the cart). So you would configure the options for the product like the example on that help page
(Ink Color);n
Black (-$0.15);-0.15
Red
Blue
Silver (+$0.45);+0.45
Gold (+$0.65);+0.65
So the first option would be text describing the option and is followed by a ;n. When displayed to the shopper they will see, in this case, (Ink Color) as the first item in the pulldown list. If they don't make a selection from the box when adding the item to the cart they will not be allowed to checkout until they select on on the shopping cart screen.
Note: these options are only available in Pro and Manager stores and are not available in Starter level stores.