Which of the following is not a model used in problem solving?

This question is asking you to separate a general thinking skill from formally defined software development models.

SDLC, the waterfall model, and the prototype model are all specific, named approaches used in problem solving within software development, each with defined stages describing how a solution is planned, built, and delivered. Critical thinking, on the other hand, is a general mental skill, the ability to analyse information logically and evaluate ideas objectively, it is not a structured, named model with defined stages that guides how a software problem is solved from start to finish.

Because critical thinking describes a way of reasoning rather than a formal development model, it does not belong in the same category as SDLC, waterfall, and prototype, which are all concrete frameworks with clearly defined steps.

Exam tip: when a question lists several named development frameworks alongside a general skill or ability, the general skill is usually the odd one out, since it lacks the defined structure that makes something a "model."