Flarum is not (yet) in the DigitalOcean marketplace, so you don't need to choose an app. You can create a blank droplet, install the webserver of your choice, eventually following one of the great guides from the DigitalOcean community. Then follow the Flarum install guide.
Alternatively you can start by installing a server management panel from the marketplace tab for a kickstart, then follow Flarum instructions for the webserver installed that way.
