Which report shows how subscribers joined a list?

The List growth report is the correct answer because it provides detailed insights into how subscribers are added to a specific list, including the source (e.g., signup form, import, integration) and the date they joined. This report is crucial for understanding the effectiveness of different acquisition channels. A Custom report allows you to build reports with specific metrics, but the List growth report is pre-built for this exact purpose. An A/B test report focuses on the performance comparison of different variations of an email or signup form, not the overall subscriber acquisition source.