Which metric would directly indicate the success of a defined marketing event?

The choice of primary conversions as the metric that directly indicates the success of a defined marketing event is highly relevant because it specifically measures the desired actions taken by users in response to that marketing effort. Primary conversions might include actions like making a purchase, signing up for a newsletter, or completing a registration form, depending on the defined goals of the event. By measuring these conversions, marketers can assess whether the event achieved its intended purpose and how effectively it engaged the audience.

The other options, while important, do not directly reflect the success of the event in terms of goal achievement. Unique opens indicate how many individuals viewed an email but do not provide insight into whether those individuals engaged further or completed desired actions. Custom attributes are useful for segmenting audiences or personalizing campaigns but do not necessarily link to success measures associated with a specific event. Revenue analytics can provide insight into the financial impact of marketing efforts but is a broader measure that might not isolate the effectiveness of a single event on its own. Therefore, primary conversions stand out as the clearest indicator of success in this context.