When designing and creating a form, you can create sections to organize fields on a form. Sections are containers that group related fields together and provide a label and a description for the group. You can use sections to improve the readability and usability of the form. You can also collapse or expand sections to show or hide the fields within them.
The other options are not valid ways to organize fields on a form. Related lists are not fields, but lists of records that are related to the current record on the form. Tabs are not part of the form, but part of the application menu that allows you to navigate between different modules. Buttons are not fields, but elements that perform actions on the form, such as saving, updating, or deleting the record.
References:
[Form sections]
[Related lists]
[Application menus and modules]
[Form buttons]

A possible reason to build a custom app is to fulfill a specific use case on internal processes. For example, you may want to digitize a manual process that is not covered by an existing ServiceNow solution, such as managing inventory, tracking expenses, or scheduling events. Building a custom app on the Now Platform can help you automate workflows, improve data quality, and provide better user experiences. The other options are not valid reasons to build a custom app. To avoid using a code repository like GitHub or GitLab is not a reason to build a custom app, as you can still use source control integration with your custom app development. To create a custom integration for a 3rd party system is not a reason to build a custom app, as you can use integration tools such as IntegrationHub or REST APIs to connect with external systems without creating an app. To replace ServiceNow base tables is not a reason to build a custom app, as it is not recommended to modify or delete base tables that are essential for ServiceNow functionality. Reference: Build Custom Apps in ServiceNow - eBook

Explanation/Reference: https://docs.servicenow.com/bundle/orlando-platform-administration/page/administer/list- administration/task/t_EditingAUIAction.html