This market will resolve according to the iOS app, ranked #1 in the United States on the iPhone Apple App Store's overall Top Charts under "Free Apps", as of 12:00 PM ET on the specified date.
To find the overall chart, click "Apps" at the bottom of the US iOS App Store app, scroll down to "Top Free Apps" and click "See All". Then under "Free Apps" in the "Top Charts" section, you'll see the list that will be used as the resolution source to this market (
https://apps.apple.com/us/charts/iphone).