• Use the audience composition report and exclude audiences that visit that site
• Identify the website’s key phrases and use them in keyword exclusion targeting
• Add the URL to the advertiser’s URL field
• Add the URL to a channel that is assigned to the advertiser’s exclusionary channel targeting

The correct answer is: Add the URL to a channel that is assigned to the advertiser’s exclusionary channel targeting

Explanation:
Exclusion: This is using a channel as a preset, excludable list of publishers where you don’t want your ads to serve. If you exclude a placement at the advertiser level, you cannot include it at the ad group level. You can exclude additional placements at the ad group level.
Read more here:  https://support.google.com/displayvideo/answer/2717490