Hello
If you are using Magento configurable products, the drop-downs are required and the buyer will not be able to add product to cart unless he/she selects needed combination from the drop-downs.
If you want to make the selection non-obligatory, you can try simple products with custom options. Though, using them you will not be able to track the inventory for variations.
Here is the article with comparison - Simple Products with Custom Options VS Configurable Products