Which type of advertisement is presented as a full-page ad before accessing a webpage?

The correct answer is interstitials. Interstitial ads are full-page advertisements that appear before a user can access a webpage. They typically load in between pages and require the user to interact with them, such as clicking to dismiss or close the ad before they can view the desired content. This type of ad is designed to capture the user's attention immediately, often featuring engaging visuals or messages.

The other types of advertisements mentioned have different characteristics. Native ads blend in with the content of the platform they appear on, aiming to match the look and feel of the surrounding content rather than taking over the entire screen. Search ads, on the other hand, are text-based advertisements that appear on search engine results pages, targeting specific keywords relevant to user searches. Social network ads are promotional content displayed on social media platforms, often within users' feeds or stories, but they do not usually occupy the entire page like interstitials do.