There is nothing wrong with the keyed answer. All three of your alternatives are correct also.
Student or Learner
--Did you enjoy the party?
--Yes. We ______ well by our hosts.
A. were treated B. would be treated
C. treated D. had treated
This was one of our test questions. The given answer is A, and I have no objection to it. But would the answer sound natural to native ears? Would "We had a very good time" or "We enjoyed it very much" sound more natural? Or is "We were well treated by our hosts" grammatically better than "We were treated well by our hosts"?
Thank you very much, Mike, for your help. (The thank and like buttons again won't work.)