Einstein Engagement Scoring is the correct answer. This tool allows you to use artificial intelligence to score your subscribers based on their engagement behavior, such as opens, clicks, conversions, etc. You can use Einstein Engagement Scoring to easily target your most loyal subscribers by filtering them based on their predicted engagement level or score. You can also use Einstein Engagement Scoring to segment your subscribers based on their predicted attrition risk or optimal send time. Google Analytics Audience is not a valid option, as this tool only allows you to measure and analyze web traffic and behavior, not email engagement. Path Optimizer is not a valid option either, as this tool only allows you to test different paths or content variations in a journey based on different engagement metrics, not target loyal subscribers. You can learn more about this in the [Einstein for Marketing Cloud] module on Trailhead.