You've got an application from an athlete, now what? Well, you've got 3 options:
Send Proposal - was the pitch what you wanted? Are they in a sport that interests you? Reach and engagement looking good? Price within your budget? Awesome, send a proposal with what you want them to do.
Send Message - need a little more information? No problem, send them a message with your questions.
Not Interested - as a courtesy so you don't keep them hanging, give a little feedback why they aren't right for this campaign. Don't worry they won't be offended and you can always invite them to a future campaign.