Retirement plan for S Corp

What are the options available for 2020 for an S Corp. (I had a client have their W'2 amended to include a 401k contribution. I do not remember why there were able to do that.) No employees other than himself.

And how did they manage to deduct the contribution from an already-done payroll?

But I don't know what options matter if they set up a 401k. Because there you are.

But I don't know what options matter if they set up a 401k. Because there you are.

Agreed. It sounds like they have one already. There you go.