The key to success on a physics test, and, I believe, the key to thinking like a physicist, is how a student reacts to such a conundrum. "The question doesn't seem to make sense. What do I do?" What do we *want* our students to do?
The ideal reaction is to write the issue as the student sees it. "This sure looks like an interference problem, but how can interference happen with a single slit?" And then, to attack the problem somehow, someway: "Well, I'm going to treat this like a double slit anyway and see what happens." I saw students using just this approach, and doing well.
The poor reaction is for a student to become offended and huffy: "Single slits don't diffract. What the heck do you want me to do, then?" "My teacher never taught me to do single slits. Can you call my school and get him fired?" "What a dumb question. We never studied this, just double slits."*
*Note that these are not verbatim student responses, just general impressions of things students have written.
It's easy enough for me and my physics teaching friends to get haughty about the poor reaction. Damn fool kids these days, they don't study, they don't listen, etc.* But never forget that it is our responsibility not just to teach the material, but to prepare our students for a comprehensive exam. The students who fight through their initial confusion to get substantial partial credit weren't necessarily smarter than the rest, but they likely had been prepared for just this sort of situation.
*Kids in our day studied, listened, respected their elders, and never texted in class.
Consider how your students might react to this question on an in-class test. Early in the school year, every student in America would want to come to the teacher's desk to say, "shouldn't this be a double slit so there can be interference?" Our job as physics teachers is to prepare the students to have their ability to ask questions eliminated. It's a physics test, not a test of how well you can read the teacher's face or coax information out of him.
I talk to my classes BEFORE our first test of the year, explaining how to handle just this sort of situation. Still, they will try to ask questions during the first test, and it's likely they will make me very angry.* The class learns that I mean business -- they have to figure out test questions on their own.
I would bet that many of the folks who became upset about the single slit were frustrated because they were used to convincing their teacher to talk them through difficult questions, and that couldn't happen on the AP exam. You and I can't possibly teach well enough that every student is fully comfortable on every question. But by testing regularly under authentic conditions, and then debriefing after each test, it is possible to train our students how to react when they're not comfortable. That's a skill that our graduates will thank us for someday.