Which clean core workstreams are recommended?

Custom code adaptation is a crucial clean core workstream when transitioning to S/4HANA, as it focuses on ensuring that existing custom code is compatible with the new system environment. This process involves assessing and adapting the custom code to eliminate redundancies and modifications that cannot be supported in the clean core. By addressing custom code adaptation, organizations can achieve a streamlined core ERP solution that leverages standard functionalities of S/4HANA, thus enhancing system performance, maintainability, and future upgrades.

In the context of the other options: simplification item checks are important but typically serve as preliminary assessments rather than a direct adaptation of the custom code; manual testing, while essential for validation, does not directly contribute to the clean core strategy; and custom code evaluation might help in identifying which pieces need attention but lacks the proactive approach that adaptation provides. The focus on adaptation directly aligns with the clean core concept of minimizing customizations and utilizing standard features, making it the most relevant choice in this scenario.