Your home decor client wants to start running mobile ads within their current campaign. Some creatives are already mobile-optimised and may be already appearing across mobile websites. What's the first thing that they should check within their Campaign Manager 360 account to ensure delivery?

The first step is to examine the trafficking settings because this will reveal if the existing campaign is already configured to deliver to mobile environments. Even if creatives are mobile-optimized, they won't deliver to mobile if the campaign's targeting or placement settings restrict it. Configuring the account (option 1) is too broad; the focus should be on the specific campaign. Setting up creatives (option 2) is important, but the question states some are already mobile-optimized, so checking delivery settings takes precedence. Modifying an existing desktop campaign (option 3) assumes the issue is with that specific campaign and doesn't address the fundamental check of delivery settings.