The Waterfall model is the classic example of a linear sequential model in software development. This methodology follows a strict, sequential progression where each phase of the project—such as requirements gathering, design, implementation, testing, deployment, and maintenance—must be completed entirely before the next phase can begin. The flow is largely one-directional, much like water flowing over a waterfall, with little to no iteration or backtracking once a phase is finished. This structured approach emphasizes thorough documentation and planning upfront.
Option A, Agile, describes a group of iterative and incremental development methods, which prioritize flexibility, customer collaboration, and rapid delivery of working software over rigid sequential phases. This is the opposite of a linear sequential model.
Option B, Spiral, is an evolutionary process model that combines elements of both prototyping and the waterfall model. It is characterized by iterations and risk management, making it distinct from a purely linear sequential approach.
Option C, Iterative, refers to models where development cycles are repeated, building upon previous versions of the software. While it has phases, the essence is repetition and refinement, not a single, irreversible linear sequence.