You plan to deploy a website to Azure. The website will be accessed by (AZ-900)

Correct answer: C) A content delivery network (CDN).
A CDN is a distributed network of servers that can deliver content, such as large video files, from locations close to the users. This reduces latency, improves playback quality, and increases the overall user experience. By caching video files at edge locations around the world, a CDN can reduce the distance between users and the content, resulting in faster load times and smoother playback.

While the other options are important Azure features, they don't directly address the video playback experience:
A) An application gateway is a web application firewall that helps protect web applications from attacks, but it doesn't specifically improve video playback.
B) An Azure ExpressRoute circuit is a dedicated, high-bandwidth connection between Azure and an on-premises infrastructure, which is not directly related to video playback.
D) An Azure Traffic Manager profile helps distribute traffic across multiple regions and endpoints, but it's more focused on routing traffic and ensuring high availability rather than optimizing video playback.

So, to ensure the best video playback experience, a Content Delivery Network (CDN) is the way to go!