Einstein STO (Send Time Optimization) Activity is the correct answer. This activity allows you to send an email at the best possible time for each subscriber based on their past engagement behavior. Einstein STO uses artificial intelligence to analyze when each subscriber is most likely to open an email and schedules the send accordingly. This can help increase the engagement rate of your email by reaching your subscribers when they are most receptive. Wait Until Activity is an activity that allows you to pause a journey until a specific date or time, or until a specific attribute value changes. Engagement Split is an activity that allows you to split a journey based on how subscribers interacted with a previous email, such as opening or clicking. You can learn more about these activities in the Journey Builder Activities module on Trailhead.