Which of the following is not a stage in product development?

The stage of "Product differentiation" is not a stage in product development. Product differentiation refers to the process of making a product more attractive by differentiating it from competitors' products in some way, such as through unique features, design, or branding. However, the actual development of the product would have already taken place during earlier stages such as idea generation, concept development, and market analysis. Therefore, product differentiation is more of a marketing strategy than a stage in the product development process.