In the Disciplined Agile (DA) framework, the architecture of a product is typically proven during the Elaboration phase. This phase focuses on validating the architecture of the solution, mitigating technical risks, and establishing a firm foundation for the project's future iterations. During Elaboration, teams work on refining the architecture, ensuring that it can support future development and will meet the project's quality requirements. The construction phase builds upon this established architecture, and by the Inception phase, the initial vision and feasibility are already defined but not yet validated. Therefore, "A. Elaboration" is the correct answer as it is the phase where architecture is thoroughly proven and refined.