Adding each send email activity to different steps in an automation ensures that each email is fully sent before the next email begins sending. The automation will wait for the completion of each step before moving on to the next one. Including a verification activity between each step is unnecessary and may cause errors. Adding each send email activity to a single step in an automation will send all emails at the same time.

Explanation
Behavioral Trigger is the correct answer. A behavioral trigger is a type of entry source that allows you to inject contacts into a journey based on their behavior or interaction with your brand. For example, you can use a behavioral trigger to start a journey when a contact makes a purchase on your website, clicks a link in an email, or fills out a form on a landing page. You can learn more about this in the Journey Builder Basics module on Trailhead.