The correct answer is option 'D' - Capability maturity model.

Explanation:
The Capability Maturity Model (CMM) is not a life cycle model but rather a process improvement model. It was developed by the Software Engineering Institute (SEI) at Carnegie Mellon University. The CMM focuses on assessing and improving the maturity level of an organization's software development processes.

On the other hand, the other options mentioned in the question are all life cycle models commonly used in software development projects. Let's briefly discuss each of them:

a) Spiral model:
The spiral model is an iterative model that combines elements of both waterfall and prototyping models. It consists of multiple iterations or spirals, each of which represents a phase in the software development process. The spiral model emphasizes risk analysis and incorporates feedback from users and stakeholders at each iteration.

b) Prototyping model:
The prototyping model is an iterative model that focuses on creating a working prototype of the software before developing the final product. The prototype helps in gathering user feedback and refining the requirements. It allows for early detection of issues and facilitates better understanding of user expectations.

c) Waterfall model:
The waterfall model is a sequential and linear approach to software development. It consists of distinct phases such as requirements gathering, design, development, testing, and deployment. Each phase is executed in a sequential manner, and progress flows downwards like a waterfall. The waterfall model is known for its strict adherence to predefined phases and deliverables.

In contrast to these life cycle models, the Capability Maturity Model (CMM) does not define a specific software development process or life cycle. Instead, it provides guidelines for organizations to assess and improve their software development processes. The CMM defines five maturity levels, ranging from initial (Level 1) to optimizing (Level 5), and provides a framework for organizations to measure their process maturity and identify areas for improvement.

In conclusion, the Capability Maturity Model (CMM) is not a life cycle model but a process improvement model, while the spiral model, prototyping model, and waterfall model are all widely recognized life cycle models used in software development projects.