Hi.. When you add characteristics you create variants of that product. So, as far as HikaShop is concerned, there are there are things that needs to be specified by choosing from the variants before it can be added to the cart.
E.G. If you're selling t-shirts that comes with only one style and colour, then it will display the "Add to Cart" button on the product category page. But, if the t-shirts offers choices in different colours & styles, then the customer will need to select the colour and style they want before adding it to cart. So in that case it shows a "Choose Options" button instead.
What you can do is style the "Choose Option" button to look like the "Add to Cart" button, but only in a different colour. This way the customer can see that there is a different and it will be the same size as the "add to cart" button, so it won't throw off your design.