To test which subject line results in the most email opens for the first email sent in an abandoned cart journey, the marketing associate should utilize the A/B Test feature in Journey Builder. This feature allows for the creation of different versions of an email with varying subject lines, which can then be tested on a segment of the journey's audience. The version that performs the best in terms of open rate can be determined through this test,and that subject line can then be used for the remainder of the journey's audience to optimize engagement.