In Act II Reverend Hale pays a visit to the Proctors farm. Since he is new in town, he has been visiting the homes of the townspeople in order to "test the Christian character of the house." Of course the Proctor's are nervous, but they believe they are innocent and try to answer his questions. His test is for them to recite their 10 Commandments. When he asks John to list them, John gets stuck and finds he can only remember 9 of them. Which one does he forget? Elizabeth quietly reminds him, "Adultery John."
Why is this scene irony? Remember incongruity it a scene or action. As the audience, we know what Hale does not, that John has previously broken this commandment with Abigail- Dramatic Irony. Elizabeth's answer is also ironic because she too knows that her husband has broken it.