How to avoid to a customer to buy a product without the model was checked?

Hi,
I am looking for information on how to avoid to a customer to buy a product without the model was checked?

For exemple: Presently, if a product has two sizes and the customer doesn't check which size he wants before clicking "add this to my cart", then the default product is added.

Is there a way to require the customer to check the size before he can proceed to "add this to my cart"?

Thank you in advance.

On the Option Values add an additional one such as:
Select One Below ...
Sort Order: 0
Default: YES
Display Only: YES

and this will require the customer to make a valid choice ...

Thank you Ajeh.

I watched in the "Option controller" and the "Option Values" and I don't see where to add:
"Select One Below ...
Sort Order: 0
Default: YES
Display Only: YES".

Could you please give me a little more details about how to do that?

Thank you very much.

As an example, in the Option Value Manager ...

Option Name: Color
Option Value: Select One Below ...
Default Sort Order: 0

Then go to your Product in the Attributes Controller and add the Attribute for:

Option Name: Color
Option Value: Select One Below ...
Sort Order: 0
Default: YES
Display Only: YES

Now when you go to the Product on the Store you will see this new Option Value: Select One Below ... as the pre-selected Attribute Option Value ... but unless the Customer picks a different choice, the Product cannot be added to the shopping cart ...

Thank you Ajeh. It works!

As I didn't need "Select One Below ..." I just checked the following "Attribute Flags":

• Default Attribute to be Marked Selected: YES

Do you know if there is a way to change the Default Attribute to be Marked Selected to YES for all products?

I watched in the "Product Type Info Page Layout options :: Product - General" to change it to:
"PRODUCT Attribute is Default - Default 1"

Unfortunately it will work only for the futur products I will insert to the catalog.

Not without knowing a great deal about your Products and Attributes ...