Extending an existing table such as the Task table when creating a new application has several benefits, such as:
You can repurpose existing fields by simply changing the label. For example, you can change the Short description field to Summary or Title for your new table.
You can use existing fields with no modifications. For example, you can use the Assigned to, Priority, and State fields for your new table without changing anything.
Existing logic from the parent table will be automatically applied to the new table. For example, you can inherit the Business Rules, Client Scripts, and UI Policies from the Task table for your new table.
The only option that is not true is d) All of the parent table records are copied to the new table. Extending a table does not copy any records from the parent table to the new table. It only creates a new table that inherits the fields and logic from the parent table.
References:
[Extend a table]
[Task table]

Application developers configure ServiceNow using industry standard JavaScript to extend and add functionality. JavaScript is a scripting language that enables developers to create dynamic and interactive web pages, as well as manipulate data and logic on the server-side. ServiceNow provides various APIs and frameworks for developers to use JavaScript to customize and enhance the functionality of their applications, such as client scripts, UI policies, business rules, script includes, UI actions, and more. References:
[ServiceNow Docs - JavaScript in ServiceNow], [ServiceNow Docs - Scripting in ServiceNow]

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