How to acknowledge copyright for figure reproduced from arXiv preprint?
The crucial points to remember here are that you have an


ethical duty to cite your source, and a
legal duty to comply with the terms of the license under which the article was uploaded to arXiv.


Those are your obligations, nothing less or (in my opinion) more. Considering this, after looking at the arXiv license information you linked to, it seems to me that the following language would be appropriate for at least one of the license options (I didn't look at all the options, the answer may vary slightly for the others):


  Figure 1. This is a very interesting figure. (Image source: [ABC15]; use permitted under the Creative Commons Attribution License CC BY 4.0.)


where "Creative Commons Attribution License CC BY 4.0" is hyperlinked to this page, and where [ABC15] refers to the bibliographic entry for the article in the references section of your paper.

Note that (as far as I'm aware) there is no need in this case to refer specifically to copyright. I also disagree with your assertion that "In general I assume it would be up to the authors to choose how they would like to be acknowledged." As I said, you have a duty to acknowledge the source of the work and its authors, and satisfy whatever legal requirements may exist pertaining to copyright or other licensing restrictions, but no other obligations that I'm aware of to use specific language that the authors or anyone else wants you to use. See here for a somewhat related discussion.