Within the context of the SAP Integration Suite, "in-app extensibility" refers to enhancing the platform's core capabilities. One primary way to achieve this is by developing custom adapters to support specific protocols or connect to non-standard systems. This is accomplished using the SAP Adapter Development Kit (ADK), which is a set of Java libraries and tools. Therefore, Java is the programming language used to build these custom components that run within the Integration Suite's runtime environment.

A. Node.js: This is a runtime for building separate applications on the SAP Business Technology Platform (BTP), typically for side-by-side extensibility, not for extending the Integration Suite's internal functions.

C. ABAP: This language is used for in-app extensibility within SAP S/4HANA (Embedded Steampunk) or for building applications in the BTP ABAP Environment, not for extending the Integration Suite.

1. SAP Help Portal - Adapter Development Kit: "The adapter development kit (ADK) for SAP Cloud Integration allows you to develop custom adapters for the integration platform. The ADK is based on the Apache Camel framework... You develop the adapter as a Java project." This document directly links Java to the development of custom adapters which extend the Integration Suite's functionality.

Source: SAP Help Portal

SAP Integration Suite > Developing an Integration Scenario > Developing a Custom Adapter > Adapter Development Kit.

2. SAP Help Portal - Tasks and Tools for Adapter Development: "To develop a custom adapter

you need the following tools: Java Development Kit (JDK)

Apache Maven

Eclipse IDE for Java EE Developers." This explicitly lists the Java Development Kit as a prerequisite for custom adapter development.

Source: SAP Help Portal

SAP Integration Suite > Developing an Integration Scenario > Developing a Custom Adapter > Tasks and Tools.