In ______ model, software is developed in a series of incremental releases.

Concept: Each prescriptive process model is identified by its defining characteristic, not by every trait it happens to share with the others. The incremental family is defined by building and delivering the product as a sequence of working releases, each one adding more functionality. The spiral approach is defined by risk-driven iteration, where every loop is organised around risk analysis and prototyping. The linear (waterfall-type) approach is defined by a single straight pass through the phases with no return for fresh releases.

Application: The phrase "developed in a series of incremental releases" is the textbook signature of the incremental model. Among the four offered choices, the member of the incremental family is RAD (Rapid Application Development) — a standard reference defines it as "an incremental software process model that emphasizes a short development cycle." So RAD is the intended answer.

Contrast with the other choices:

• RAD — incremental delivery is its defining purpose: the system is split into modules, built in short time-boxed cycles, and shipped as successive working increments. This matches the stem exactly.

• Spiral — it does iterate and can ship prototypes each loop, but its defining purpose is managing risk; incremental release is incidental to it, not its identity. That is why it is not the model the stem describes.

• Sequential — a single straight pass through the phases (a waterfall-type flow); it does not return to earlier phases to deliver new releases, so there is no series of increments.

• Concurrent — organises activities or states that run in parallel; it is not characterised by delivering software as incremental releases.

Cross-check: Since "Incremental" itself is not listed, the correct pick is the option whose defining trait is incremental delivery. RAD is the unique member of the incremental family here, so RAD is the best and only fitting answer.