There is a whole series of events where the original rules were altered. Originally the pigs lived under a constitution of 8 commandments. One of them was about alcohol which was forbidden. In this scene it was altered to be allowed in moderation.
Sleeping in beds was also forbidden. That was altered to sleeping in beds with sheets. So you have to choose between B C or D which all have truth in them. The other animals all know that there is something wrong, but they cannot use the law to right things. The pigs have foreseen that they might resort to this and have changed the law so it now allows them to do what they are doing. Nobody has a copy of the original constitution.
So what do we pick? Barley does not have to be used for beer. Some of the animals (like the horses) are vegetarians and barley would be a treat. It is true that the pigs had the barley planted, but never for the benefit of others. B is true, but it is not ironic.
They prohibited alcohol when they took over the farm. That is a fact.
What is ironic is that they discovered that alcohol in limited amounts was enjoyable and so they modified the constitution to suit the fact.
That makes C the answer, which I like as an answer. It is not only ironic, it shows the hypocrisy of the pigs. They didn't legalize drinking entirely, just the right to have a little bit. Who defines that? They do.
Apparently the answer from outside sources is D. So that's what you should go with.
