The correct answer is D - Facilitate a just-in-time exploration of the functionality by the customer and the team.
Agile promotes emergent planning and just-in-time elaboration. When a customer cannot clearly define how a feature should function, collaborative exploration at the point of need (just-in-time) ensures shared understanding, timely feedback, and minimized waste.
From the PMI Agile Practice Guide:
"User stories are deliberately kept light to encourage conversation. These conversations often occur just before the story is scheduled for development-known as just-in-time elaboration." (PMI Agile Practice Guide, Section 5.2 - Backlog Preparation and Refinement) Andy Crowe also explains:
"Agile teams defer detail until the last responsible moment. This supports flexibility and better understanding through collaboration. Facilitating exploration sessions when needed helps clarify expectations and reduce uncertainty." (Andy Crowe, PMI-ACP Exam Prep, Chapter 5 - Value Delivery) Why other options are less suitable:
* A may be helpful for technical uncertainty, but the issue here is lack of functional clarity from the customer.
* B (iteration 0) is more of a preparatory stage; it is not typically used mid-project.
* C suggests decomposition but without resolving the lack of understanding; it assumes the problem is granularity, not clarity.
Hence, the correct agile action is to engage collaboratively at the point of need: D
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