ServiceNow Studio is used to develop and manage application artifacts within a scoped application, including server-side logic that runs when database operations occur. "Customizing system behavior through database actions" points directly to server-side automation triggered by record operations such as insert, update, delete, and query. The platform feature designed for that purpose is Business Rules, which execute on the server in response to database events and can validate data, set field values, enforce policies, or initiate additional processing. While Studio can also include other file types such as Client Scripts, UI Policies, and ACLs, those are not primarily described as "database actions." Client Scripts and UI Policies are primarily UI and client behavior controls, and ACLs control security access. Business Rules are the canonical mechanism for database-triggered behavior and are the best match to the question's wording. Therefore, the file type aligned to customizing system behavior through database actions is Business Rules (sys_script).

Tables that extend a table inherit the parent's fields. Extending a table means creating a child table that shares the same columns and business logic as the parent table. For example, the Incident table extends the Task table, which means that all fields defined on the Task table are also available on the Incident table. Extending a table allows for reusing existing fields and behaviors without duplicating them on multiple tables. Reference: Table extension and classes

The welcome screen appears every time a new application is created through the Guided Application Creator.
The welcome screen provides an overview of the steps involved in creating an application, such as defining the app name, scope, and tables, configuring the app user interface, and publishing the app. The other options are not true about the Guided Application Creator. The global scope option is turned off by default, as it is recommended to create applications in their own scope for better security and performance. A scope application user role is not automatically created, as the user can choose to create one or use an existing role for the app access control. Default access controls are not automatically created, as the user can define the read, write, create, and delete permissions for each table in the app. Reference: Guided App Creator

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.
Reference:
[Extend a table]
[Task table]