Final answer:
The collapsing rule failed because two or more life events were in the detected phase. The number of proximity days specified may not have been appropriate.
Step-by-step explanation:
The collapsing rule failed because the benefits administrator received an error stating that two or more life events were in the detected phase. The collapsing rule is designed to combine multiple life events into one if they occur within a certain proximity of each other. The question provides options A, B, C, and D to explain why the collapsing rule failed. From these options, it is likely that the number of proximity days specified was not appropriate, as either option A or B would have triggered the collapsing rule successfully.