Which Amplify buying option allows advertisers to verify that their ads are reaching a specific demographic via Nielsen?

Demo Guarantee is the correct option because it specifically offers Nielsen demographic verification for ad campaigns. This allows advertisers to ensure their ads are seen by their intended audience based on Nielsen's robust measurement. FixedRate is incorrect because it refers to a set price for impressions or views, without demographic verification. Biddable is incorrect as it describes an auction-based buying model where advertisers bid for ad placements, also without inherent Nielsen demographic guarantees.