The 'Select Options' button is displayed when your product have options that need to be selected before a customer can add it to the cart. Customer cannot add such products to their cart before they have selected the options they want.
When there is no option to select, the "Add to cart" button is displayed directly as the product can be added without needing other steps.
Does that make sense?
> How can I add (add to cart) button before (view details) button or to make the add to cart button to be side by side with view details button?
> Why is the button 'View details' displayed instead of 'Add to Cart'?