For developing custom applications to integrate with SAP S/4HANA Cloud Public Edition, SAP recommends using tools available on the SAP Business Technology Platform (BTP). SAP Business Application Studio is the standard cloud-based Integrated Development Environment (IDE) for professional developers, supporting pro-code development of Fiori apps and full-stack cloud applications. SAP Build is SAP's unified low-code/no-code offering, enabling citizen developers and business experts to create applications, automate processes, and build business sites with minimal coding. Both tools are central to SAP's "clean core" side-by-side extensibility strategy for S/4HANA Cloud.

C. SAP HANA Cloud: This is a database-as-a-service (DBaaS) used for data storage and management, not a tool for building the application logic or user interface itself.

D. SAP Cloud Portal Service: This service is used to build and manage central access points (portals) to applications, not for developing the actual custom applications.

1. SAP Help Portal, SAP Business Technology Platform: "SAP Business Application Studio is a cloud-based development environment, tailored for the efficient development of business applications for the SAP Intelligent Enterprise." This is the primary pro-code tool for extensions. (Source: SAP Help Portal, What Is SAP Business Application Studio?)

2. SAP Help Portal, SAP Build: "SAP Build is a unified low-code offering on SAP BTP that empowers users of all skill levels to easily create and augment enterprise applications, automate processes, and design business sites." This positions SAP Build as the key low-code tool for creating applications. (Source: SAP Help Portal, What Is SAP Build?)

3. SAP Learning Journey, Extend SAP S/4HANA Cloud in the Public Cloud: This official learning path explicitly covers side-by-side extensibility for SAP S/4HANA Cloud. Unit 3, "Developing an Extension Application," details the use of SAP Business Application Studio for pro-code development, while Unit 4, "Low-Code/No-Code Extensibility," focuses on using SAP Build Apps. (Source: SAP Learning, Learning Journey: Extend SAP S/4HANA Cloud in the Public Cloud, specifically Units 3 & 4).