Recombination between DNA segments question
So, apparently the "trick" to this question is in the wording. "between A and B" appears to mean one crossover somewhere in the B segment, which could destroy (delete) a gene in the B sequence if present at the cut.
The answer involving the horseshoe "trick" implies that the chromosome has looped so that the A and C sequences match up, an extremely unlikely condition. Then a crossover occurs while the loop is in place, also an extremely unlikely event.
Another problem is that the horseshoe solution apparently implies that it is a single chromatid that is looping to itself and forming the crossover. That's weird. I could more easily visualize a different solution: One chromatid is aligned tail-to-head with respect to the other chromatid (head-to-tail). This would allow the corresponding A and B segments on the two chromatids to match up, thus allowing a crossover point anywhere (except inside B) to result in an inversion of B. This is also weird.
These two solutions are much weirder and unlikely than a simple crossover inside B: Picture the two chromatids aligned a little off target so that one is cut at the start of B nearest the A segment, and the other chromatid is cut at the other end of B, nearest the C segment. The result is an uncomplicated deletion of the B segment from one of the chromatids and duplication in the other (no inversion). This scenario seems at least as valid as the "correct" choice given in the practice exam.
While I'm glad to have learned the "trick" in this question for when I take the exam, I consider this question "bad".