What license should I include in my WP style.css file?

I’m a new theme author and currently finishing off my first WordPress theme. I’m checking all theme files before submission and my eye fell on the information in the style.css file.

At the head of the style.css is information about the theme author etc. There’s also the option to add a license:

License URI: 

I searched on the internet but can’t find information on how this should be handled. What should I add to the license parts when I use Themeforest to publish a WordPress theme? Or should I skip this?

Many thanks!