Must Select Any one Option in Product

I have a product with two option.
The two options are Rent and Buy.
The prices of book are changed according to the option selected.
I have kept both options WITHOUT "required" fields so that user can select any option according to their choice.
But now the users are able to add to cart without selecting any option.
I want the user to select any one of the option before adding to cart.
How to check if the user have selected any one of the option?
Please help me
Thanks in advance.

Couldn't you just have one "select" option with Rent and Buy as the options, if you make it required then the customer would have to choose one or the other.