Transparent Data Encryption (TDE) is the appropriate method for ensuring that the underlying database files are encrypted at rest. TDE encrypts the database files themselves, protecting the data in case the physical media is stolen, while allowing the data to be viewed in clear text by authorized users within the system.
* Transparent Data Encryption: Encrypts data at rest, ensuring that the database files are secure.
1: Salesforce Transparent Data Encryption

The newly created data model in Contact Builder can be used for segmentation in the following applications:
* Email Studio (A) - Email Studio can use data extensions and attribute groups from Contact Builder for segmentation.
* Journey Builder (C) - Journey Builder can leverage the data model created in Contact Builder for creating targeted journeys and audience segments.
References:
Salesforce Marketing Cloud Contact Builder
Journey Builder Data Integration

In Marketing Cloud, the Contact Key should be represented as a Text data type, even if it is a numeric value.
This is because using the Text data type avoids issues related to numeric value formatting and allows for greater flexibility in handling the values.
1: Salesforce Contact Builder Best Practices

The operational order of the Contact Delete process for Marketing Cloud is as follows:
* Delete initiated - The process begins when the contact delete request is initiated.
* Suppression phase - During this phase, the contacts are marked for deletion and are suppressed from further communications.
* Final Deletion - The contacts are permanently deleted from the system.
References:
Salesforce Marketing Cloud Documentation on Contact Deletion
Contact Builder in Marketing Cloud

To validate that a parent-level data extension can be accessed by a child business unit, the developer should ensure thatthe data extension is in the Shared Data Extensions folder and the query includes the ENT. prefix (A). This setup allows shared data extensions to be accessible across different business units.
References:
* Salesforce Marketing Cloud Shared Data Extensions
* Query Activities with Shared Data Extensions