The ______ model is preferred for software development when the requirements are not clear.

Explanation

The evolutionary model develops an initial implementation, exposes it to the users, and then refines it through many versions until an adequate system emerges — exactly the strategy Sommerville recommends when requirements are poorly understood at the start. RAD is a high-speed adaptation of the waterfall that assumes requirements are well understood and the scope is tightly constrained, so it is unsuitable here. RUP is a heavyweight, use-case-driven process framework rather than an answer to vague requirements. The waterfall model is the worst choice, because it freezes the requirements in a document before design begins. Hence the evolutionary model.