The default option will be given the same ID as the product.
so for example, if you look in the ProductOptions table, it has the following columns:
ProductOptionID - Primary key
ProductID - Foregin Key, relates to products.ProductID
OptionID - Foreign Key, relates to options.OptionID
GroupingID - Foreign Key, relates to optionsGroups.optionsGroupID
If the value of the ProductID column and the OptionID are the same, that option is the default for that product.