Which software life cycle model is based on iterative prototyping?

The phrase "based on iterative prototyping" points to a life-cycle model whose core mechanism is to build a prototype, evaluate it, and feed that evaluation into the next round of development, repeating the build-evaluate-refine cycle across several passes instead of moving through the process only once. It is true that prototyping is itself often iterative — for example, evolutionary prototyping keeps refining a single prototype toward the final product — so the Prototype model can look like a valid choice. What singles out the intended answer is an extra defining trait: repeated prototyping loops that are each governed by explicit risk analysis and formal planning of the next loop.

Barry Boehm's Spiral model is exactly that model. It structures a project as a sequence of loops ("spirals"); each loop sets objectives, builds and evaluates a prototype (or other risk-reduction artifact) for that stage, formally analyses risk, and only then plans the next loop. In standard software-engineering references the Spiral model is the one canonically described as coupling the iterative nature of prototyping with systematic, risk-driven control, which is why it is the intended answer here.

• Waterfall model: runs requirements, design, implementation, testing, and deployment once, in strict sequence, with no repeated loop back to build or revise a prototype.

• Agile model: also iterative, but each iteration is scoped to deliver a usable increment of the real product rather than a formally risk-assessed prototype loop.

• Prototype model: repeatedly builds and refines a mock-up to clarify unclear requirements, but is not organised around formal, risk-driven loop planning; it is the closest distractor to the intended answer.

• Spiral model: across every loop it builds a prototype or other risk-reduction artifact, evaluates it, and formally assesses risk before planning the next loop — the risk-driven, iteratively prototyped process the description points to.

Therefore, among the given options the Spiral model is the correct answer.