In which of the following items is situational irony best defined? a contrast between the two leading characters in a story a contrast between what is expected to happen and what actually happens a parallel between the beginning of a story and the end a humorous scene in an otherwise serious story

I THINK, technically speaking, that the answer you're looking for is:
Option B: a contrast between what is expected to happen and what actually happens

However, irony itself can be particularly difficult to accurately define, so I would recommend taking the technical definition (Option B) at face value, and accepting that there's a bit more to it.
Hope I could help, let me know if I was correct or not!