A software development life cycle (SDLC) model is a plan for how a project moves from an idea to working software and then to maintenance. The choice of model matters more than the choice of programming language, because the model decides when requirements are frozen, when testing happens, and how change is handled. Two opposite answers dominate exam questions. The waterfall model is strictly sequential: requirements, design, implementation, testing and maintenance happen one after another, and each phase is signed off before the next begins. It fits only when requirements are stable and well understood, for example a payroll system whose tax rules are fixed by law. Agile methods do the opposite: they deliver working software in short iterations, welcome changing requirements, and test continuously. They fit products whose requirements will change, for example a new campus app whose features are discovered from student feedback.
Worked example — choosing a model. A college wants an attendance system. The rules are known (subjects, sections, biometric devices, report formats) and will not change mid-project. Waterfall fits: freeze the requirements, design, build, test, deploy. A startup wants a food-delivery app but does not yet know which features users want. Agile fits: ship ordering first, learn from users, then add tracking, offers and reviews in later iterations. The exam trick is that no model is "best" in general; the right answer always follows from the requirements and the risk in the question.