First of all you need to create the Option category, and then the Option from your Back Office (Products>Option).
Then, go to Products and click on the specific product. On the pop-up window on the right, you scroll down and will see "EXTRA AND OPTION". Click on Add an option and you will be able to choose the category and to set it for that product.
You can decide if you want to have an automatic pop-up window anytime you are taking an order and you choose that specific product.
If you choose "limited" options, make sure you enter the maximum number of choices possible.
Refresh your app, and you will see the option offered at the moment that you are creating a new order.
That's it!