You need to unify the different data sources within Microsoft Dynamics 365 Customer Insights through the data unification process.
After you selected Run on the Merge page, you notice that you still need to rename a merged field.
What is the fastest way to perform the correction?
To correct the name of a merged field quickly, you should view the details of the running job and cancel the job from the side pane. This approach allows you to make the necessary change to the field name without waiting for the current merge process to complete, resulting in the fastest correction and restart of the merge process.
Both B & C can do the job, but option C is quicker.
In a forum thread discussing similar situations, users confirm that canceling and restarting the job is the quickest way to make changes to field names during running merges. See https://learn.microsoft.com/en-us/dynamics365/customer-insights/data/data-unification-merge-tables.
C is fastest
The answer is absolutely C, especially considering how long the Merge process can take to run. It's taken upwards of 30-45 minutes for me at times, and occasionally an hour when I had very large data sources. Canceling the job to make the correction is going to be a LOT faster than waiting for it to finish incorrectly and then going in to make the edit and having to run it all over again, anyway.
i would go with B