A/B Testing
Why do A/B testing?
A/B testing is done to test multiple email copies and see which of the copies is working best.
An email copy's performance is measured by its opens, (clicks) unsubscribe, and most importantly --replies.
Pro tip: Using email variations also improve your email deliverability.
This is because sending emails with different variations will make it difficult for spam filters to block you.
How to do A/B testing?
A/B testing can be easily done by creating multiple variations to an email step.

Fill out the email subject and body with the new variation and click Create Step.

For a more detailed guide on email variations, you can check out this article -- Variations.
Which email variation is working best?
Each email variation has its own stats.
Clicking each email variation, you can see the open, click, reply, and unsubscribe rates of the variation.
The stats also show the number of prospects that the variation has been sent to.
These stats will help you determine which email copy is the most effective.
How to stop sending an email variation that is not performing well?
The goal of A/B testing is to rule out the best email copy.
If an email copy is not helping you receive replies, then you can stop sending it by pausing the email variation.
To pause an email variation, just click the triple-dot icon of the variation and then click Pause variation.
Warning: Avoid deleting email variations as doing so might cause journeys to run into an error.
When a journey runs into an error, the prospect won't be able to receive further follow-ups.