They never ask an open essay question that applies to a handful of works, so if you know and understand HoD, you will probably be able to base an essay on it. AP tests are difficult and stressful, but you need to stay focused and calm. They will be worded in such a way that you can argue any number of positions. You can generate specific supporting material for your essay on HoD by thinking about imagery, word choice, and opposition. The first two are pretty straightforward.
Opposition refers to pairs of elements that contrast sharply.
The obvious oppositional pair in HoD is black and white, and you can find a great deal written about it in the postings below. In addition, pay attention to oppositions of characters to one another, of characters to settings, of style to subject, and so on. Once you attune yourself to this idea, you can find them everywhere. Your real job is to decide how they support whatever theme you've picked for your essay. Your essay will be scored but not corrected, and the difference is critical.