Messenger Stories is the correct placement because the Stories format is specifically designed to be a full-screen, vertical, and immersive experience on mobile devices. This aligns directly with the advertiser's goal of using an ad that covers the entire mobile screen for maximum engagement. Ads in Stories use the 9:16 aspect ratio to fill the screen between organic user-generated content.

A. Facebook Marketplace: Ads in Marketplace are displayed as sponsored listings within the scrollable shopping feed, not as a full-screen takeover.

B. Facebook Feed: Feed ads are integrated as posts within the main content feed and are constrained to the post's dimensions, not the full screen.

C. Instagram Explore: Ads appear in the Explore feed after a user taps on a photo or video, maintaining the feed's browsing format rather than a full-screen one.

1. Meta Business Help Center, "About placements in Ads Manager." This guide categorizes placements. "Stories and Reels" (which includes Facebook Stories, Instagram Stories, and Messenger Stories) are distinct from "Feeds" (which includes Facebook Feed and Facebook Marketplace). The guide's structure and descriptions implicitly define Stories as the full-screen format. (Meta Business Help Center, Article ID: aBu334k3m3mX3kY)

2. Meta Blueprint, Course "Select Ad Placements" (350-101). The official course material states: "Stories placements on Facebook, Instagram and Messenger let you use immersive, full-screen ads to capture your audience's attention." This directly confirms that Stories placements fulfill the advertiser's requirement.

3. Meta for Business, "Stories Ads." This resource explicitly describes the format: "Stories ads are full-screen, vertical ads that appear between organic Stories... With full-screen creative, you can immerse your audience in a way that feels native to their Stories experience." This provides a clear definition of the format's primary characteristic.