The question asks which option is not a part of the strategies under the Rainfed Area Development (RAD) scheme of India. Let's analyze each option provided:

1. Diversified farming: This is a strategy that promotes the integration of different farming activities. It helps in enhancing resilience to climatic variability and improving the livelihood of farmers. It is indeed a part of the RAD scheme.
2. Optimizing canal irrigation: The RAD scheme focuses primarily on rainfed areas, where the dependency is mainly on rainwater due to the lack of irrigation infrastructure. Canal irrigation is more related to irrigated areas and is not a focus of the RAD scheme. Therefore, this option is not a strategy under the RAD scheme.
3. Adaptation of cluster-based approaches: This approach involves group farming techniques where farmers come together to implement best practices and share resources. It is a key strategy under the RAD scheme to increase efficiency and productivity in rainfed areas.
4. Soil health management: Ensuring soil health is crucial for sustainable agriculture, especially in rainfed areas. This strategy is aimed at maintaining soil fertility and is a part of the RAD scheme.

After evaluating all the options, it is clear that Optimizing canal irrigation is not a strategy in the Rainfed Area Development scheme as it is more applicable to irrigated farming practices. Hence, the correct answer is Optimizing canal irrigation.