The validation must happen when a Contact is saved because the missing field is Region on the Contact record. Salesforce validation rules evaluate data before a record is saved and return an error when the formula evaluates to True.

The app builder can create a validation rule on Contact that checks the related Account’s Customer Status field and requires Region only when the Account is marked High Priority.

Example logic:

AND(

ISPICKVAL(Account.Customer_Status__c, " High Priority " ),

ISBLANK(Region__c)

)

Why others are incorrect:

A is wrong because the missing field is on Contact, not Account.

B is wrong because Customer Status being required does not require Contact Region.

D is wrong because it would require Region for all Contacts, not only High Priority Account Contacts.

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