In an iterative lifecycle, in which stage are the solution requirements explored and defined?

In an iterative or Agile lifecycle, requirements are not defined entirely upfront but are explored and refined just-in-time, immediately prior to implementation. The stage where the detailed solution requirements (functional and non-functional) are elaborated, modeled, and defined—ready for construction—is the Develop solution stage (often referred to as the Implementation or Construction phase of an iteration/sprint). The detailed work of defining, modelling, and validating the specific requirements happens as a preparatory step for the actual build, distinguishing it from the preceding planning phase (C) or the upstream strategic work (A, B).

(Reference: BCS Requirements Engineering – Lifecycle Models, Iterative Development)