Which of the following is necessary for an attendee to download the event app?

The ability for an attendee to download an event app is reliant on the App URL. This URL serves as the link that directs users to the specific app for the event. When attendees click on this URL, they are led to the platform (such as the App Store or Google Play) from where they can download the app directly to their devices.

While other options may play important roles once the app is downloaded or being used, they are not necessary to initiate the download. For instance, the Event ID might be required later for logging in or accessing specific event information after the app is installed, while user credentials are typically related to user accounts or features within the app itself. A promotional code might be used for discounts or special access, but it does not affect the initial download process.