Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window org.wso2.carbon.identity.challenge.questions.api.user.challenge-1.0.2.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.